We had five or so players join during HFC as the result of them calling it quits on being able to get into mythic with the guild they were running (small server/lack of talent).
The plan was for them to run a second team under our guild banner, which they did successfully up until a point. While we had 7/7M EN on farm, they couldn't get past Ilgynoth. In the end, despite them not improving, pretty much everyone who wasn't in that five was told they'd no longer be raiding, and the five (some of whom had "officer" spots at that point) joined our main roster. Some of them made it undoubtedly stronger, but there was one of the new officers from that guild who was just atrocious. I'm talking less damage than me over a night of Odyn progression despite me playing tank & him playing DPS
The whole thing left a sour taste in my mouth. Firstly because 20+ people were left homeless because they called it quits on their team again after being frustrated with the lack of progress, & secondly because we were seemingly doomed to take a deadweight character or two to the raid. I ended up leaving for a top 200/300 guild (for this & other reasons) & saw out the rest of Legion with them, which was great fun
Now I'm back in that old guild however, as the slight adjustment to leadership plus a much better roster has led to a much more enjoyable raiding experience with a bunch of lads I get along with, so I guess it all worked out in the end.