Yeah, mostly a problem with larger quantities. I feel pretty confident that the vast majority of people on this forum use rectangular brooders, I do. The concern is that something will panic them (it often doesn’t take much), they will pile up in a corner, and some smother. It does happen.
There are a lot of things that can happen, and actually have. A piece of space junk might fall out of the sky and hit your house. You might have a fender bender on a trip to the grocery store. The sun might shine here today. The odds of these things happening can vary quite a bit, but they all have happened.
The possibility of chicks smothering by them piling up in your brooder is possible, even in a round brooder, but it’s not very likely. It’s something I’m not concerned about.