Here's some reasons I've heard. Tl;dr, install Ubuntu if you want.
1. Snap. Ubuntu has snap, which is a way to distribute packages (like apt or flatpak). Ubuntu is kind of forceful about snaps, and a few packages, like Firefox, will be installed using snap by default. There might be some disadvantage I don't know about, but I get the sense people mostly don't like how Ubuntu tries to force you to use snaps, and how a lot of the same advantages can be achieved with flatpak (sandboxing, security, etc.). I've also heard the Ubuntu CEO imagines moving towards a system completely using snaps in the future.
2. The Ubuntu Pro ad(s?). Ubuntu Pro is a service that provides extra updates, and updates for old LTS versions of Ubuntu. With the free tier, you can connect up to 5 machines in a 24-hour period (i.e., you can have infinite connected machines, but only 5 active in one 24-hour period). If you don't connect Ubuntu Pro, some command line things (apt I think) will nag you to connect Ubuntu Pro to get those updates.
3. Amazon. Ubuntu previously partnered with Amazon, adding Amazon searches to the desktop. They faced heavy backlash, and eventually removed the feature. There were some privacy concerns associated with this as well.
4. Canonical. Ubuntu is developed by a private company, Canonical, who mostly makes money by selling support for Ubuntu Server to corporations. Some people just don't like a corporation controlling their FOSS OS. Some worry about Canonical collecting their data. I haven't looked into this much, but I haven't seen anything that suggests Ubuntu collects very important data.
Tiny addition, they also try to trick you into signing up for their newsletter thing when you download an ISO, but you don't have to in order to download, they just delay the download start by a second or two.
Some people may disagree, but I think you should install Ubuntu if you want to.
My biggest issues is with snaps, but if you're new and don't care as much about the inner workings of your system, this might not matter much.
Canonical has done some stupid things, but they are nowhere near the level of Microslop. If they do cross the line for you in the future, migrating to another OS isn't really that hard once you understand Linux.
That said, I personally prefer Mint over Ubuntu, so staying with Mint is also a good option. If you do try Ubuntu, Kubuntu (Ubuntu with KDE) also has a Windows-like interface, or Ubuntu Cinnamon.