It's personal. Some will call one spec the hardest, some will call another spec the hardest. It all depends on what makes a class harder for you, be it tracking dots, managing rage/energy/focus, managing self buffs, holy power/embers/soul shards etc etc.
Regardless, the right addons make most specs quite trivial.
Blizzard seems to translate hard with how many buttons you have to push for your rotation.
Which led to button bloat and the need of 20-30+ keybindings dependig on the class.
Personally I think encounters should be hard to master but not classes.
Where is the difference between pushing 1,2,3,5,1,2,3,5 or 1,1,1,2,1,1,1,2 ... after a week or so both feel the same you don't even look at what you do anymore.
I believe the topic specifies what specc is harder to master.
Simply knowing your rotation well doesn't mean you're a master of your class: If this assumption was true you'd see many more guilds progressing alot better.
It's about the mastery of timing, aligning certain abillities / spells / potions with trinket proccs / legendary meta gem proccs, what talent works best for what type of encounter or job assigned to you / min maxing, situational awarness, ingenuity etc.
I've about played them all, and Demonology easily takes the cake. While all specs get easy over time, with enough practice and muscle memory, the Warlock specs still are the most complex to properly get into and maximize your DPS.
Well as a lock I always liked the fact that demo was kind of hard to master considering the amount of différent CDs to align correctly and in the right order and the switch between meta and normal form but in fact it's not that hard and even when you suck you can push high numbers.
Playing a warlock I can say that when you need a skill unit of 75 to master affliction, the number for Destruction is 90 and for Demo it is 100. Keep in mind Im talking about exceptional performance. It is much easier to push out average numbers as destro than it is for affliction for example, but to play at your spec's max potential, Destro is much more demanding. And playing Demo averagely is still easier than playing Destro masterfully etc.
I think all of the Warlock specs are difficult, enough so that I ended up deleting my lvl 74 lock as it felt like more work than fun to do everything right. But I don't feel informed enough to say that Warlock end-game play is different than anything else.
My main has a Feral off-spec and a lot of the difficulty can be trivialized with a mod that has timers on it. I use Cat's Eye. If you don't like playing with a mod, then you can forget it. Don't even bother. You have too many different things you have to watch, they're all in different parts of the screen and by default it's very hard to see when things are about to expire. It requires energy management, combo point management, manual buff and debuff maintenance, twitch proc reactions, timing and mid-term planning, all while being a melee class with many useful hybrid abilities. By default, you get one cooldown which just makes everything happen even faster.
Enhancement has a rotation that's miles long and lots of cooldowns, but you can just line up all your buttons on your bar according to priority and it makes everything easier. Not so with Feral, which has relatively few actual buttons to push but if they're not placed perfectly, it hurts a lot. it has you doing the Rogue combo point dance while also watching for a window every 30 seconds where you do extra damage, a la Fury Warrior, plus weaving in hardcast heals.