Mostly the lack of e2ee by default (like seriously, even WhatsApp does this), data collection for targeted ads (since relatively recently), closed-source server-side, and providing data to law enforcements despite claiming not to in their FAQ.
Also them constantly breaking backwards compatibility by introducing useless garbage (like that textured background when replying to sb with premium) is kinda annoying. Like seriously, they could've just extracted the text instead of "this message is not supported by your version of $client_name"
There are good (usable) flake templates you can just
nix flake init
, tho