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There is not any hero in the game that can turn the tide in a war by himself, unless you get a very good hero VERY early (the chances of getting a hero are based on your scales, especially your luck scale).

There is a big zone between "what is the point" and "overpowering" and every hero should be in that zone. A hero won't really provide any flavor if you can't do anything with him besides shuttle troops around, maybe his text is interesting but you really need a gameplay effect to appreciate it.

There's more than one way to become a hero - one way is to do ordinary things very well, and another way is to do extraordinary things. There is room for both in the game. Taking the example of Ulm, I agree a powerful mage would not make sense, but a powerful priest might (given that Ulm has lousy priests normally, but doesn't really have a problem with religion other than lack of interest). Or a vampire in the middle era or a mage with the forge bonus in late era, heralding or recalling a different time. These are ways they could be different from the usual without really being out of place. A hero with a really strong siege or fortification bonus wouldn't really be different, but could certainly be better without just having tiny stat bonuses like most Ulmish heroes.