I've had some pretty terrible experiences trying to to collaboratively edit word documents in Office 365. You get all sorts of corruption with people bumping into one another. Big groups, small groups, it doesn't matter, at some point you'll see some malarkey.
I’m with you. I’ve had some pretty terrible experiences with this in the past as well.
I haven’t had many issues over the last 6-12 months, though, and as far as I can tell, they continue to improve.
Don’t get me wrong - I’m not a fan of MS products and as I said, I think Google still has the edge in collaboration. But they’ve come far, and it wasn’t that long ago that those collaboration features were unusable or non-existent.