It's a departure from the desktop UI and it made the whole thing much clunkier.

I will be fair, moving the button to the bottom row is more convenient if you have a lot of servers to scroll up through and use the DMs a lot, I don't care about that as much.
What I do care about is how the rest of it changed. Can't just quickly swipe into the DM list and back any more, if you exit the DM that's it, you gotta click on it again. Switching between DMs and servers is more clunky, a bunch of UI I was actively using is broken up. They broke the search function, and the new way to check channel participants (click on the title) is uncomfortable as hell compared to old "just swipe to the right".

Personally I hate the new UX even if some bits are an improvement. It's just too much stuff to change all at once and not for the better.