I really like this game! I've just gotten started with the campaign, but here are some first impressions.
The tactical combat is still the star of the show, of course. With the change to a science fiction setting, guns are more important. Many units now have an overwatch mode like in XCOM. This can be countered by staggering weapons like grenades or sniper rifles. They knock enemies out of overwatch and also drain movement points for the next turn. One thing that I like a little less about Planetfall is that it went with a hit-chance system. In AoW3, cover, range penalties etc. reduced the amount of damage, but your basic attacks always hit. (Yes, halflings were an exception. They also were annoying.) I liked that because it made combat less random than in AoW2. So I'm a bit disappointed that Planetfall changed it back. Still, it's not that bad: if you have a reasonable hit chance, most misses will become grazing shots that still do half the normal damage. Another thing I was skeptical about were the unit mods. There's now a wide variety of ways to customize your units. I thought I was going to encounter lots of units with surprise abilities that I didn't expect, or forget which of my identical looking soldiers has what upgrade. But on the tactical map, every unit has a little marker to indicate what kind of upgrade it has, so it's all good!
The biggest change to AoW3 (besides the whole science fiction business) is that there are a lot more city management options. On the one hand, that means more fiddly stuff: every city now has workers to assign to different resources -- food, energy (= money), production and science, the usual -- like every other 4X game. I really didn't miss that in AoW3... On the other hand, the new sector system is really interesting!
The map is chopped into sectors, consisting of multiple hex tiles each. There can be only one city per sector and only in a fixed location. At certain population milestones, a colony can annex a new sector. That gives it access to any special structures in there, like an old biodome for additional food or a power plant for energy. You then choose how to exploit that sector by selecting one of the four resources. An exploited sector has a level between 1 and 5. At low levels, it increases your income per turn by a small amount and gives you additional worker slots: at the start, a colony has only four worker slots for each resource, and sectors are a way to increase this cap and specialize the city. On higher levels, sectors increase the income produced by each worker.
To level up sectors, you need to research and build certain improvements in your colonies. For example, one gives a boost to all the city's production sectors. Some are terrain-dependent: "Mountain Exploitation" gives one level to energy and production sectors in mountain regions, for example. Getting sectors to level 3 is fairly easy with the right terrain, but you have to choose to specialize in that type of sector at the expense of other types. Getting a sector all the way to 5 requires enormous amounts of research and production, and perfect terrain for that type of sector.
On top of that, every sector can have up to one special improvement whose effect scales with the sector level: For example, a food sector can have a building that makes its colony grow faster, or a building that lets it export food to other cities more efficiently. A research district can be specialized towards military or civilian research.
Planetfall has a classical tech tree, unlike its predecessors. Or rather, a collection of tech trees. Some parts are determined by your faction or your Secret Technology (like class in AoW3), other parts are common to all, like the Economy tree. That's where you find all the sector upgrades, and it has a pretty interesting structure which allows you to skip many technologies. It's actually like a deterministic version of MOO1's "tech ladders" where researching a technology on one tier unlocks all techs on the next tier. There are lots of ways to approach this depending on how far you want to specialize in what sector type, and what terrain you have available. Maybe it'll turn out that there's one right way to do things, I don't know. But I'm having fun playing around with it.
In my previous game, I went all-in on research sectors in city ruins. In my current game (with the Kir'ko space bugs), I'm gearing up my starting colony to be a huge food exporter to quickly build up further cities.
The "fluffier" parts of the game are also great! Graphics are neat, and I love the soundtrack, especially the battle themes. The setting is very interesting: you're rebuilding on top of the ruins of the Star Union, a fallen galactic empire. Ancient precursor aliens are a pretty stale concept by now, but in Planetfall, the precursors are human, and the empire is still in living memory for those who spend a lot of time in cryo pods or otherwise have an extended lifespan. The ruling class of the Star Union is still around, as a minor faction of cybernetic zombies that terrorize the countryside with demands for tribute and style themselves as the last guardians of civilization.
The tactical combat is still the star of the show, of course. With the change to a science fiction setting, guns are more important. Many units now have an overwatch mode like in XCOM. This can be countered by staggering weapons like grenades or sniper rifles. They knock enemies out of overwatch and also drain movement points for the next turn. One thing that I like a little less about Planetfall is that it went with a hit-chance system. In AoW3, cover, range penalties etc. reduced the amount of damage, but your basic attacks always hit. (Yes, halflings were an exception. They also were annoying.) I liked that because it made combat less random than in AoW2. So I'm a bit disappointed that Planetfall changed it back. Still, it's not that bad: if you have a reasonable hit chance, most misses will become grazing shots that still do half the normal damage. Another thing I was skeptical about were the unit mods. There's now a wide variety of ways to customize your units. I thought I was going to encounter lots of units with surprise abilities that I didn't expect, or forget which of my identical looking soldiers has what upgrade. But on the tactical map, every unit has a little marker to indicate what kind of upgrade it has, so it's all good!
The biggest change to AoW3 (besides the whole science fiction business) is that there are a lot more city management options. On the one hand, that means more fiddly stuff: every city now has workers to assign to different resources -- food, energy (= money), production and science, the usual -- like every other 4X game. I really didn't miss that in AoW3... On the other hand, the new sector system is really interesting!
The map is chopped into sectors, consisting of multiple hex tiles each. There can be only one city per sector and only in a fixed location. At certain population milestones, a colony can annex a new sector. That gives it access to any special structures in there, like an old biodome for additional food or a power plant for energy. You then choose how to exploit that sector by selecting one of the four resources. An exploited sector has a level between 1 and 5. At low levels, it increases your income per turn by a small amount and gives you additional worker slots: at the start, a colony has only four worker slots for each resource, and sectors are a way to increase this cap and specialize the city. On higher levels, sectors increase the income produced by each worker.
To level up sectors, you need to research and build certain improvements in your colonies. For example, one gives a boost to all the city's production sectors. Some are terrain-dependent: "Mountain Exploitation" gives one level to energy and production sectors in mountain regions, for example. Getting sectors to level 3 is fairly easy with the right terrain, but you have to choose to specialize in that type of sector at the expense of other types. Getting a sector all the way to 5 requires enormous amounts of research and production, and perfect terrain for that type of sector.
On top of that, every sector can have up to one special improvement whose effect scales with the sector level: For example, a food sector can have a building that makes its colony grow faster, or a building that lets it export food to other cities more efficiently. A research district can be specialized towards military or civilian research.
Planetfall has a classical tech tree, unlike its predecessors. Or rather, a collection of tech trees. Some parts are determined by your faction or your Secret Technology (like class in AoW3), other parts are common to all, like the Economy tree. That's where you find all the sector upgrades, and it has a pretty interesting structure which allows you to skip many technologies. It's actually like a deterministic version of MOO1's "tech ladders" where researching a technology on one tier unlocks all techs on the next tier. There are lots of ways to approach this depending on how far you want to specialize in what sector type, and what terrain you have available. Maybe it'll turn out that there's one right way to do things, I don't know. But I'm having fun playing around with it.
In my previous game, I went all-in on research sectors in city ruins. In my current game (with the Kir'ko space bugs), I'm gearing up my starting colony to be a huge food exporter to quickly build up further cities.
The "fluffier" parts of the game are also great! Graphics are neat, and I love the soundtrack, especially the battle themes. The setting is very interesting: you're rebuilding on top of the ruins of the Star Union, a fallen galactic empire. Ancient precursor aliens are a pretty stale concept by now, but in Planetfall, the precursors are human, and the empire is still in living memory for those who spend a lot of time in cryo pods or otherwise have an extended lifespan. The ruling class of the Star Union is still around, as a minor faction of cybernetic zombies that terrorize the countryside with demands for tribute and style themselves as the last guardians of civilization.
Still dangerous until the new empire was strong enough, but ultimately brushed aside.
Hopefully you are correct and that is just an effect of the campaign set up. A 4x type game just does not work well if the opponents do not expand and challenge you for territory.
From left to right: units, colony center buildings, sectors (selecting exploitations, and sector buildings), workers, and an overview of constructed buildings and terrain features in the colony center's sector. Also note that sector level upgrades are built in the 
Sometimes they are obvious, and sometimes they do sneak up on you, as you described.