Design. Thats all
Other than that i love it, but the design it just too confusing and not clean enough, if you compare the design to other competitors like trello and more closely Asana you'll see it how confusing the UI is. Adding another issue i hate is the mobile app. These are the only things i hate the most. But there more things i love about clickup but thats another question to discuss ;)

It's difficult to get much done using keyboard commands alone. I wish they made it possible to navigate and use the app using only the keyboard.

At the moment (02/2020) User- and Tag-Management is rather simple, but works.

A bit slow but that's fine.

The UI could feel smoother. Trello definitely has the upperhand here

It's too pretty, that it hurts my eyes - probably it's their typography choices.

It's an incredible tool that's 10x more efficient than JIRA, but they ship features at such a break-neck pace, sometimes things break. For example, at the moment, their burndown widget is broken for me and so is comment scrolling on Firefox.