Admittedly, it's been a few years and I'm coming due, but let's see what I can remember...

• apt will brick itself if it gets interrupted mid transaction with no clear recourse apart from a total reinstall, so try not to get greedy and Ctrl+C if it looks like dpkg is hung
• trying to install any software that isn't already packaged explicitly for Ubuntu is a nightmare because there is no equivalent of the AUR for people to push build steps to and you're quite often left guessing what dependencies you need to install to get something to compile
• snapcraft, need I say more? Firefox takes several minutes to start up, we don't talk about disk usage, installing a package with apt will sometimes install the snap version anyway requiring a Windows-registry-edit-esque hack to disable, and the last time I checked in, the loop devices it creates didn't even get hidden in the file manager.
• I've also definitely encountered my fair share of bugs and broken packages which are always fun to fix