They are just poorly designed. From HP, cost, pillars, to how they can be placed on top of each other. The whole design is just bad. They can be useless, to completely busted in certain situations or maps.
If anything needs a future rework... it's walls.
While they are at it they could give the Euro civs maybe some more unique re-skins to fit the civ building set better... Ya know to make the more casual players care.
walls give no kill xp which is bs, and make hand cav impossible to use, meaning they can get away with one unit type for 100 wood. They also make rushing 10X harder which is almost enough to completely deny an entire playstyle especially when combined with CM.