I pretty much agree with your tiers.
I do think that America may be as strong or stronger than some of the "worth having a look" tiers, because
a) that combat bonus is a whole-game thing;
b) the combat bonus is tied to your home continent, which you are guaranteed to have access to, unlike a foreign continent - still a bit situational;
c) Rough riders come at a decent time to be pushing for a final win - although nothing upgrades into them.
The real problem is that they've been played twice and are not very interesting.
I've not played India or Kongo. Kongo have never made much sense to me. Their UU has detterent potential, but I think it is too early and the wrong class to be much use on offense. On a typical map, and one lacking the "rush" civs, crossbows will be in the field by the time expansion is over and the warring begins. Possible if you really, really want to go early GG and offense - but Greece may have grabbed the GG...
I don't think I got the best out of Brazil when I played them in SP. They're kind of interesting if you think a cultural victory is a real possibility in MP. But in the world of chops, apprenticeship and industrialism having to keep rainforest around is kind of meh. And their unique district just isn't one I can see spamming in MP. Better GP recruitment, kind of OKish, but you have to keep competing for lots of them to get to this ball rolling. The MG is powerful but situational. Range 3 bombardment can be useful on quite a lot of maps though...
I don't know about India. Again, I feel the Varu is going to be up against knights and crossbows by the time serious war starts, but may still hold some value with it's higher strength and the special ability. Shame about the movement though. I can see stepwells being useful, but they compete with chops for builder changes. Early they are an alternative to a granary, later they can keep you growing up towards size 10 more easily for that fourth district. The enemy war weariness could potentially be fun, both as a deterrent as a drain on anyone you're conquering, but it's really only great in a one-on-one situation. You get the better of a long war, but anyone else at peace is better off still.
Of the three, I kind of like India, but haven't any particularly strong feelings.
A lot depends on what kind of victories are practical in Civ VI and how you get there. It's been pretty much domination, domination, domination (in PBEM2 those missionaries were travelling on the stirrups of cossacks). Peaceful religion is pretty much impossible now that players understand the mechanisms. Science seems tricky to me - very dependent on getting the right Great People. Cultural? Maybe.
So, domination depends on having access to good units. This is primarily a science thing, IMHO, although culture can help boost the science. More directly, culture can get you to corps and armies - although Nationalism seems to have been broken by the patch, with corps now actually only available at mobilization, which thus cannot be boosted
. Meh, maybe Brazil is the most relevant then for the district bonii - campuses, commerce for the upgrade gold, maybe faith for stuff too? And hope to be able to get Chichen Itza. If the mapmaker has given you lots of jungle and Kongo is not competing for it...
I do think that America may be as strong or stronger than some of the "worth having a look" tiers, because
a) that combat bonus is a whole-game thing;
b) the combat bonus is tied to your home continent, which you are guaranteed to have access to, unlike a foreign continent - still a bit situational;
c) Rough riders come at a decent time to be pushing for a final win - although nothing upgrades into them.
The real problem is that they've been played twice and are not very interesting.
I've not played India or Kongo. Kongo have never made much sense to me. Their UU has detterent potential, but I think it is too early and the wrong class to be much use on offense. On a typical map, and one lacking the "rush" civs, crossbows will be in the field by the time expansion is over and the warring begins. Possible if you really, really want to go early GG and offense - but Greece may have grabbed the GG...
I don't think I got the best out of Brazil when I played them in SP. They're kind of interesting if you think a cultural victory is a real possibility in MP. But in the world of chops, apprenticeship and industrialism having to keep rainforest around is kind of meh. And their unique district just isn't one I can see spamming in MP. Better GP recruitment, kind of OKish, but you have to keep competing for lots of them to get to this ball rolling. The MG is powerful but situational. Range 3 bombardment can be useful on quite a lot of maps though...
I don't know about India. Again, I feel the Varu is going to be up against knights and crossbows by the time serious war starts, but may still hold some value with it's higher strength and the special ability. Shame about the movement though. I can see stepwells being useful, but they compete with chops for builder changes. Early they are an alternative to a granary, later they can keep you growing up towards size 10 more easily for that fourth district. The enemy war weariness could potentially be fun, both as a deterrent as a drain on anyone you're conquering, but it's really only great in a one-on-one situation. You get the better of a long war, but anyone else at peace is better off still.
Of the three, I kind of like India, but haven't any particularly strong feelings.
A lot depends on what kind of victories are practical in Civ VI and how you get there. It's been pretty much domination, domination, domination (in PBEM2 those missionaries were travelling on the stirrups of cossacks). Peaceful religion is pretty much impossible now that players understand the mechanisms. Science seems tricky to me - very dependent on getting the right Great People. Cultural? Maybe.
So, domination depends on having access to good units. This is primarily a science thing, IMHO, although culture can help boost the science. More directly, culture can get you to corps and armies - although Nationalism seems to have been broken by the patch, with corps now actually only available at mobilization, which thus cannot be boosted
. Meh, maybe Brazil is the most relevant then for the district bonii - campuses, commerce for the upgrade gold, maybe faith for stuff too? And hope to be able to get Chichen Itza. If the mapmaker has given you lots of jungle and Kongo is not competing for it...
It may have looked easy, but that is because it was done correctly - Brian Moore
They still seem kind of dull, though. I can't think of anything I'd want to try out in a test opening with them.
. They're great on tiny maps, or as defenders / deterrent if you don't have iron (unlikely on an RB map); they can be useful as support units for crossbows. They can also be useful for capturing city states early (not something I tend do do in SP much, but highly viable in MP). But the Kongo ones don't have a strength or attack bonus (compare with legions or the Macedonian district/city killer), so I don't rate them particularly. 
per turn. War weariness is doubled for civs at war with Gandhi.
, +1 housing. An additional +1 
, needs no resource. Adjacent enemy units get -5 strength. This does not stack and doesn't seem to affect units garrisoned in cities. Upgrades to Tanks.
I think I missed it because the two debuffs didn't fit into the textbox at the same time. City strength seems to take only the base strength of the unit into account, e.g., fortifying inside a city also does nothing, and the debuff just doesn't apply to cities. When the unit is attacking out, it should be debuffed, but to test it, I would need India to besiege me.