1.you could fake progress your skills/magic by casting/performing activity by yourself
2.enemies leveled up with you which completely destroyed the whole point of leveling and the whole RPG aspect of it
Eventually these 2 fundamental flaws proved to be too much so I just stopped playing mid game.
I heard that same thing happened to Oblivion so I never even tried that one.
I really wish they could work on Fallout 4 instead, lasers and guns dismembering in VATS never gets old, but now it could have much better animation, laser disintegration effects and consistently coherent camera view.