I've never really touched on these type of topics but I would want to argue that doesn't this all come down to stat+skills? I mean I guess it would be sad to see an offense capped 70-speed lynn get outdps'd by an offense capped 50 speed scythe evie but I have yet to see something like that. (but I hardly play this game anyways >.>)I think the issue that people want addressed is not the "higher difficulty should have higher reward" concept, but rather the actual potential of certain characters. You can have similar stats to another character and play perfectly - but the other character is still able to dish out considerably more damage even if they've messed up.
Rewarding certain classes by making them stronger due to their skill ceiling isn't how vindictus works. Even back when Kai first came out, he was broken and everyone knew how to play him (with bows). Scythe evie back then was considered one of the hardest characters to play because not alot of players could get past 40 attack speed which made it difficult to find an opening, she's a class that deserves some respect. So, how difficult to play a character just determines whether or not you should play it, that's all.