the major problem is the raid difficulty being only 1 mode and its tuned heavily towards the near impossible for even 40 players of avg skill.
retail has multiple difficulty modes from lfr to normal to heroic to mythic and has nearly the same amount of modes for all the other content in the game.
so yes while the raid having 1 mode is more leaning towards vanilla , it is harder by a very long shot from anything vanilla or even hm 4 naxx last two bosses difficulty.
people are fine with mechanics being a wiper but not fine with an artificial enrage where the boss goes godmode and owns the entire raid cause they couldnt meet the impossibly tuned for 20 man dps checks.
even full 40 mans of players i would def consider above avg struggle and dont even clear last boss and have a very hard time with the 2 before that one.
the simple fact is on every other version of wow and on every raid prior to this one in sod in particular , there was always a way to tone it down to gain gear in another mode or whatever to pass the harder content.
being locked behind the pitiful amount of loot to gear the entire raid with no other way to get the loot other than a weekly turn in to even be able to hope to pass later bosses in the instance is daunting or down right something that makes people want to quit and i dont blame them.
if i wasnt fortunate enough to be running with the guilds im running with i would have no hope to pug or get any loot and i would be hard stuck.
have some sympathy.