An Ubuntu dist upgrade on a machine of mine once forgot how to read an encrypted root disk. That its own installer had set up originally, and the config of which I'd never touched (the machine as a whole had approximately no customization, in fact—it was just a box I had hooked up to a TV, nothing special going on). This was just a one-version jump, mind you.
[EDIT] Oh and there was that release in the late '00s, I think, when they pushed out PulseAudio way too early (shoulda never done it at all...) and it caused your whole X session to crash if your browser loaded any Flash content at all. Which was still in wide use at the time. How the shit that got through QC, I'll never know. There's no chance, in that year, that someone testing or developing it hadn't tried to load HomeStarRunner or something, or had a Flash ad try to load. Hell I think YouTube might have still used Flash on Linux at the time.
I love Linux but I've had way more stuff break on me with upgrades then I have with any Windows or Mac updates and the stuff that does break is usually the real painful stuff like video card drivers where I can't even boot to a terminal because I can't see anything. I usually end up doing a clean install.
I'm pretty confidendent in saying that I've had Windows, OSX, Debian, Red Hat, Fedora and Ubuntu release upgrades all result in some kind of breakage that has taken time to fix.
I think the only platforms I've not had comparable levels of breakage with are FreeBSD and Solaris, but then I only ever did one upgrade in both cases, so maybe I just got lucky.
It's great if you've never had problems, I feel envy for you, but some small % of users will have problems due to the sheer number of different hardware and software configurations out there.
Hell, I haven't even managed a single upgrade of OpenWRT without losing a bunch of additional configuration / packages.
I wouldn't go that far: I've had issues with every major version upgrade so far and I've been using Linux for over 25 years. My experience with Ubuntu has been a bit bumpier with Ubuntu than other distros on desktops in particular.
There's always at least one driver (or default) that has changed just enough to break something requiring research to fix and usually a couple of packages that have been removed from the repos. So I always figure on an afternoon to deal with any immediate (i.e. booting) issues and a bumpy few days as I have to work out any substitutions.
A ubuntu upgrade bricked a virtual server that I had relied on. Network access was so broken that I ended up exporting the data, and killing the machine.
We run MacOS in my corporate environment and updates are universally dreaded. Our CTO (an excellent programmer/technologist) refuses to update his laptop (and rejected a machine upgrade) because of the breakage of applications with each OS update. Granted we don't have the device driver failures that are encountered in Linux because Apple defines the machine specs, but changes to their privacy/security settings breaks all of our apps every single time.
In 10 years I've never had a dist upgrade issue.