Of course, there are always some imbalances, and in this case, the clear favorites are high-level units + high-defense units; with any of these combinations, you can switch off your brain. But where isn't that the case?
In Heroes 3, everything is destroyed by high-speed creatures, like phoenixes. In Heroes 4, there's no point in tactics if you level up uber-heroes with combat skills or gather a doomstack of vampires. In the fifth game... Well, in the fifth game, it's probably a bit more difficult, although even there, in the late game, you can become overpowered if you're a necromancer with five castles or an orc with upgraded rage.
Unfortunately, a good game can always be broken, and the task here is to make it so that it's interesting in a normal working scenario, rather than always. Of course, for this, you need to make a suitable base, and not one that immediately filters out a lot of options as impossible or useless.