In my experience (6 campains played, all with 4 heroes) The OL only has the distinction of being "OP" in the begining.
I've noticed that the majority of the "OL is OP" are still playing in there 1st or second campain.
Once the heroes have figured the game out, what skills work and which ones to avoid, the heroes become OP.
Again I've noticed that the majority of the "Heroes are OP" have played multiple campians.
It comes down to this.
My group has put this game on the shelf for one reason.
No one wants to play the Overlord.
It's an uphill struggle to win at best, and it's nowhere close to as fun or easy as playing a hero.
I saw a fun idea. Heroes pick archetype at random and then their character at random. This forces them to reevaluate how they go about the game. They can't pre plan synergy.You are forced to rediscover the game.