off the top of my head:

- all useful interactions are hidden behind useless slow animation and multiple additional clicks

- photo selection is an awkward double scroll

- stickers are tied to external apps, cannot be searched, take ages to load when you switch between sets

- link unfurling and file previews are hit and miss, often take ages to happen

- pinned chats are useless awkward circles that take up space instead of being actual pinned chats

- continuously broken on MacOS (photo selections, unread status synchronizations, keyboard shortcuts etc.)

And it's a key app they have on two platforms.

Compare that to Telegram which exists on 5 platforms and web, developed by just a handful of people, and has everything and more.