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Joined: Aug 2015
You can't buy the research or casting skill for gold. You can only buy buildings that produce some but they all produce it over time, not instantly. Summoned units are also much harder to buff, Warlord, Adamantium, Holy Armor all don't apply to them. Furthermore summoned units are vulnerable to Life magic while normal units are immune to it, so one more realm can damage them effectively.
But most importantly, those strategies cost 8 books of the same realm. Armorer's Guild unit strategies cost 2 books (2 Life for Healing) and all the other picks can go towards boosting the strategy. Sapher had Chaneller to counter healing costs, Runemaster to counter unit curses, 2 Chaos for Flame Blade to accelerate early treasure hunting, Myrran to have access to draconian bowmen, Warlord to make the units and bowmen stronger (warlord wouldn't work for summons anyway and Tactician doesn't make bowmen deal damage faster) and even 1 Nature book for web, resist elements and opening up even more trade possibilities.
If you do 8 books for Spiders, you still need 2 Life for healing, leaving you with only 2 more picks to spend freely.
Needless to say it's possible to do the same if you only pick 2 Nature books and randomly find Giant Spiders (or research lots of commons and it shows up again at random) but you can't build a strategy around that, if it does happen that's pure luck. If you actually try you'll have it happen like 10% of the games and then you still need to deal with casting skill not being readily available for gold and summons not getting buffs and early uncommon summons generally not being as powerful/durable as the top tier normal units.