I've had bad tinkering break my system before, but never had an update break it irreversibly. The closest would actually be on Silverblue itself, when an update to the kernel was using different signing keys that cause the system not to boot. Fortunately it was simple, I selected the previous deployment and I was in (on a non versioned OS I would have selected the previous kernel which most are configured to retain the last few). A quick Google revealed Ublue had a whole kerfuffle and after verifying it was legit, I enrolled the new certs into my MOK.
Although one time on Arch I had installed an experimental version of Gnome from one of their repos, and was pleasantly surprised when that version finally released and I removed the experiment repo and did an update absolutely nothing at all broke. Nothing.
I've seen it a few times in passing and always assumed it was like, a tech demo or proof of concept.