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A hero should be heroic, and many of them simply aren't. Ulm's heroes are hardly better than normal commanders. They don't necessarily have to be super-powered but they should give you something you normally don't have, or be a better version of your regular, commonly used units. That's why Etimmu is such a good hero, because powerful undead are good units to start with, but he's slightly better, plus you can get him earlier than you can summon anything comparable. Ulm's heroes are better than their normal commanders, but the problem is that their normal commanders are not particularly great.

Plus, mortal combat units don't tend to have particularly long lives, another drawback to the Ulms. Mages and immortals are much more valuable as heroes because they last longer. A mortal combat hero needs to be *really* good in order to be of value because otherwise all he will accomplish is dropping a bunch of expensive equipment in some field somewhere.