NIN/MNK difficulty?

[Shamox]

Probably because you can't tell if a NIN is good or bad by only looking at its personal dps.

Beside being pretty much hardware dependant there's actually way more than simply doing your rotation correctly and using everything on cooldown, you have to adapt and you can optimize your whole rotation (and ninki rotation) depending of each fight, you also have to map every trick attack correctly to get the most rDPS out of it, same goes for enmity management.

Monk gameplay overall feels empty, of course there's gonna be optimization just like any classes and you'll have to get mantras where it's needed yet it wont be as demanding.

Overall what these numbers shows is that optimization is less rewarding depending of each class.

Imo in terms of difficulty a melee classment would probably be NIN - DRG > SAM - MNK.

I wouldn't really take any of these FFlogs stats as facts since there's many factors that actually impacts on your dps and just as I said you don't judge a class difficulty nor a player's skill only by its dps.
(Aaand if I really were to check them I wouldn't go as down as the 50th % percentil tbh.)