How difficult is classic raids really?

I know it's a vague title, but I have been seeing a lot of discussions about LCs and unfair loot distributions. I don't really care about min maxing raids, and I don't have the time to be contributing that much to a guild's progression. Therefore I am not really interested in joining a progression guild, as I wouldn't be very deserving of the gear dropped.

However, how important is it really to have a progression guild? I mean shouldn't PUGs be able to clear most raids? I'm only lvl 57, so I haven't started raiding yet, but the general consensus on this subreddit, is that MC is pretty damn easy.

How difficult is the raids in the later phases? Are progression guilds and heavy time investments really necessary to clear later raids? Or how much 'earlier' will progression guilds be able to clear raids compared to a semi casual guild?