What does it look like when a rolling distro "breaks"?

I've been running debian sid on a spare drive for a while and I really like it, but I question it's stability long term, I don't want to distro hop until I know my next distro WILL work. When people talk about rolling distros like arch or debian sid being unreliable and "breaking" after a "bad update", what do they mean? Would the system just refuse to boot? Would it be more of a minor bug like a part of my DE breaking? In most cases how fixable is this "breakage"? My current main distro is Kubuntu 19.04.