Yes, all races are well-balanced! THAT’S WHY THE GAME IS SO INTERESTING!
To avoid making unsubstantiated claims, I’ll give an example:
Humans have retraining, while Necromancers can turn units into undead. Mages have mini-artifacts, Elves have great archers and a sworn enemy. Demons have gates, and the Inferno has chaos. And so on and so forth.
Of course, a lot depends on the situation, skill, simple luck, and many other things that are simply pointless to discuss.
I personally love Necromancers the most, but this choice is not based on their strength, but simply because I like the undead!
I don’t know how to play all races, but I’ve fought against all of them. And based on my experience, I can say that it was equally difficult to fight against all of them!