I'm not sure what their specific pain points were with Microsoft, but I definitely had a few. Forced updates were a big one for me. I've always had irregular and slow internet, and having my computer be unusable for an hour or two because I hadn't updated for a few days just didn't work! They were always bugging me to sign up for a account as well, and also resetting my default programs and reverting the telemetry settings.

If Canonical wasn't so Microsoft-esque with strange decisions. The good old Amazon ads in the old versions, or the ad for Ubuntu pro in the terminal, old habits of cooking homegrown solutions instead of contribution to existing projects, or snap being centralised to their closed appstore and their push for this tech. They're not evil, but personally I don't like to trust them. Hell Canonical even had a Windows mobile called Ubuntu Touch, which now is a separate project. Linux Mint is just easier in this regard.

Ironically I started my linux journey with Manjaro, which has its own pile of crap and still survives in one of my systems after 4 years and 3 failed updates by power outage.