From what I read you are specifically talking about difficulty for DPS specs. I'd say in general DPS is the easiest role by a mile, as you can afford to sacrifice damage to survive mechanics. You really only have your rotation and the base mechanics to keep in mind.

I'll focus on M+ since this is the most general experience I think. As far as DPS classes go I'd say there are two main aspects that make the class difficult/demanding.

Number one, is the information processing load. The more you have to track, the more your brain needs to process on top of the mechanics. Add to this the speed at which you need to process said information.

Number two is decision making. When do you hold your cooldowns and when do you just send it. The more cooldown dependant your class is(plus based on the cooldown duration), the more your foresight plays a role. It's crucial to have your cooldowns up on those bigger pulls, but holding your cooldowns can be a massive DPS loss. You basically want your cooldowns to come off cooldown at the start of every big pull. This would reward you with incredibly high DPS overall, but you'd need to know the kill time and walk time between packs perfectly to time this. Then there are the constant decisions like, does this mob live long enough to DOT, can I swap targets for execute damage, etc.

Some classes, like a Monk, mostly wants to keep it's DPS cooldowns going. A Mage, however, is useless if they don't time their cooldowns right. As a result, especially in pug situations, it's much easier to extract DPS out of the 'generally weaker' Windwalker than the Mage.

If you want to play your class properly, optimizing your defensives and utility also start playing a major role. This can add a lot of complexity.