I've never bothered with the Trending page. The sinister algorithm seems to be working perfectly for me, so the Home screen only recommends stuff I'm interested in.
Most annoying one: Playlist management is extremely limited. You can't select multiple videos in one playlist and delete or move/copy them at once to another playlist.
That one is a big bother for me. I have a large playlist of music videos saved and shifting tracks around is very clunky. You're stuck with moving them to the top or bottom of the list, or veeeeeerrrrrry slowly dragging them up or down.
A related issue is that, whenever a video gets deleted for whatever reason, it doesn't tell you what the video was. I can understand them not wanting to make it easy for you to search for the same pirated content again, but it's blinking annoying.
The biggest annoyance is adverts randomly playing during the middle of videos. Thankfully, I've got ad blockers on my PC, so it's only an issue on my phone.