I kind of know why I don't like it:

It's javascript-heavy and likes to behave more like an app than a good-old website.

It loads the messages dynamically -I never liked infinite scrolling-, can be slow and unreliable, especially on patchy internet connections, hijacks ctrl+f for its own search function and its UX is a weird combination of minimalism and fanciness.

Its setup is also more convoluted than I like.