Well, you're not quite unstoppable until the last game. The enemies in the games leading up to it should still pose a threat. These games have at least one enemy that can be quite intimidating if your stats aren't reasonably high. The cheetaurs in the 1st, giant scorpions in the 2nd, necrotaurs in the 4th, and so on. The demons that were built up to be your number one foe in the 3rd? Not so much, especially since, unless you're a fighter, you don't need to fight them at all to get past them. Hell, the dinosaurs were harder to kill, and those are incredibly common enemies that you can battle not long after you start the game.
Also, while the demon wizard had quite a bit of foreshadowing, the actual confrontation with him was a bit of a letdown, not helped by the fact that he looked like a guy wearing a Halloween mask. Not quite the endgame I was hoping for.