For me, it was a combination of Ubuntu breaking upstream and introducing its own unnecessary systems.

I had a few issues caused by Ubuntu that weren't upstream. One was Tracker somehow eating up lots of CPU power and slowing the system down. Another was with input methods, I need to type in a pretty rare language and that was just broken on Ubuntu one day. Not upstream.

The bigger problem was Ubuntu adding stuff before it was ready. The Unity desktop, which is now fine, was initially missing lots of basic features and wasn't a good experience. Then there was the short-lived but pretty disastrous attempt to replace Xorg with Mir.

My non-tech parents are still on Ubuntu, have been for some twenty years, and it's mostly fine there. I wouldn't recommend it if you know your way around a Linux system but for non-tech, Ubuntu works well. Still, just a few months ago I was astonished by another Ubuntu change. My mom's most important program is Thunderbird, with her long-running email archive. The Thunderbird profile has effortlessly moved across several PCs as it's just a copy of the folder. Suddenly, Ubuntu migrated to the snap version of Thunderbird, so after a software update she found herself with a new version and an empty profile. Because of course the new profile is somewhere under ~/snap and the update didn't in any way try to link to the old profile.

Then there were stupid things like Amazon search results in the Unity dash search when looking for your files or programs. Nah. Ubuntu isn't terrible by any means but for a number of years now, I'd recommend Linux Mint as the friendly Debian derivative.