Honestly I played vanilla and I am playing classic again and after doing mythic raiding and high keys, I would describe the difference as apples to oranges. What makes classic "hard" is the sheer amount of grinding and lack of gear. But now we have all this gear and complex stat requirements so the dungeons and raids seem easy because we all learned how to gear up quickly. We are doing raids at 15-30 ilvls higher than what they are built for. In example a +15 is trivial if you give your team all 460 ilvl gear.
Whereas in vanilla you didn't have that gear in "harder" dungeons and raids.
Tldr, harder to get gear in vanilla, but otherwise not hard at all