I think people overestimate a lot of content and problems. The muscle memory just doesn't disapear and often a good observation/research/understanding is a key.
Over the years the I havent found that many if any encounters that I would say "of crap, this is difficult - on PERSONAL level or execution complexity" but becouse you arent raiding alone and the complexity factor of encounters increases due to human factor included exponentially.
On a side note, more and more mechanics over the years become not complicated but often confusing - I think this without proper understanding is nowadays bigger factor of becoming a better player. The faster you understand the problem, the reason for it the better. I won't lie that sometimes coming to a boss in world 20-30 race some mechanics were confusing as fuck in the begining, think partial reason for this was wording, language difference and barrier, another one was were "naked" ability descriptions in built in boss ability info.
I have a history of (I supposedly wrote a list down here but I decided to not post it. I would get accused of bragging here likely) so nvm.
The biggest and hardest challange for me was to play feral competitivly in 2011-2013 as the class/rotation was extremly complicated, unintuitive, required extreme timing, understanding, fast fingers and not fooling around, with some encounters it was mental challange to sustain few hours a day every day.
Luckily at some point we had some tools to make life easier but in the end it was marely a guide, not a solution.