Or, more generally, "stable" is supposed to mean things don't change within the release apart from security and bug fixes. "Unstable" is apt to change, both in terms of package versions or features and in terms of the way the system is structured. It can temporarily have broken dependencies or breaking changes as well. Or at least that's how it was when I used to run unstable years and years ago.

I stopped running sid not because of any instability or unreliability in the included programs themselves but because unstable required more active administration: apart from temporarily broken dependencies or upgrade paths, it made sense to stay informed on potential breaking changes. Stable(r) releases or distros rarely require almost any active care nowadays apart from installing security updates.

Again, my experience with sid was years and years ago but I don't suppose its fundamental nature as the active development branch has changed.

Unstable means that updates could change things about your system that you rely on. This could be a package getting removed, but it could also be a package upgrade that necessitates a change to your workflow or code running on the system..