I'm sure there are as many reasons as there are people who dislike Ubuntu, but here's a few:

• They injected internet ads into search
• To many outside of the community if they have any familiarity with Linux on a desktop, it's with Ubuntu which kinda places it in a position to newcomers as being Linux itself rather than one particular flavor
• It is very opinionated about look and feel and usability: i.e. their custom launcher and Snaps
• It's popular
• It has a reasonably large user base so there's more opportunity for people to find things to nitpick over.

Overall it's fine. I've used Ubuntu, Mint, Puppy, DSL, Arch (btw), Fedora, and Debian. I can do pretty much anything I need to on any of them. I've got my preferences about the correct balance between useability, upgrade schedule, and customizability.