I think any game with stat points and armor bonuses can be min maxed to death. I've seen people 1 shot final bosses in Diablo. Is "that" OP, sure but it also takes someone designing any ENTIRE build around a certain skill. I always spread out my skills because I play 90% solo. If I was playing with 1 specific goal in mind for my build, I could certainly go in a direction that would be considering OP, but at the same time, your character is TRASH at a lot of other things. It all depends on what you want to do. The only skill I think "was" OP in my experience was parkour in A21.
Parkour USED to be OP, you could literally fall from any height and not die. WHILE cool, it actually completely ruined the game for me because I could just nerd pole to the top of any POI and then jump off and run home with zero fear. In a MP setting, anyone with maxed parkour and speed boosts to run speed had a crazy unfair advantage compared to other players. They nerfed it, it's still really powerful, but it's not super hero level powerful. Anytime a certain skill or item puts a character into "Super hero" status mobility/damage/immunity wise the game ceases to be fun for the player, because the challenge is gone, or that player becomes a nightmare to deal with on MP servers. When you break a core game mechanic, that needs to be looked at. I love a good power fantasy, but if it goes to far, it's no longer fun in my opinion.