Tac is somewhat right - by the time you level up the hero, they may no longer be able to turn the tide of battle, even if they have an ability...

It depends on who we're talking about. A Wizard, after gaining a Special Ability (SA), will immediately become powerful, transforming from a helpless weakling. The question is, who will wait for them to level up? Moreover, without magic, they will have problems even with neutral creatures. Unless they find a Forest of Ancient Sylans with a mentor nearby.

A Demon, while leveling up to gain a SA, will become stronger with each level, as there are almost no useless abilities on the way. And after gaining it, they will receive a very significant boost.