In my 15+ years of raid leading (mostly pugs) I have consistently had the most problems with anyone who says this. They are Always the first to rage quit.
Generally my groups have a couple different skill levels of players, and the groups are usually competent but not completely-overpowering-the-fight-competent.
It’s frustrating because I hate to turn down qualified players but there’s so many fights where communication is key. Target swaps, dodges, healing CDs, doesn’t matter if a player has killed a few twelve times, if they want to do it the current one they need to be working with the team.
There’s only one or two other groups of players I’ve had as much trouble with. I’m not going to mention them by name, but they weren’t an issue when I was fighting Ragnaros in vanilla.