For me Fire Emblem Warriors lost against Hyrule Warriors some appeal for the normal enemies all look alike. Do I have this time around the ones ion red uniforms or in violett or whatever? In Hyrule Warrios Gorons looked different than skeletons different than Hyrulian army different than Bokoblin and so on.
Also the bosses, the big monsters especially were much better in Hyrule Warriors. That has to do with the use of items to defeat them. In FE Warriors it was basically a mounted unit. It also seems easier, which might be subjective. Harder mode makes the enemies only harder to kill, not cleverer in taking the battlefield.
On the other hand the pair-up dynamic in FE Warriors was great.
Most of the enemies in FEW are either humans or a specific looking monster, so you are correct in its lack of variety.
I found the bosses to be easy hack and slash. I was actually shocked I beat the game when I did (I figured the final boss would be a lot harder and later in the game).
As to your last point, I agree. FE has always had great pair ups and that continued with this game. I really liked having a caster/archer as my support character for combos.