This is not an exhaustive list, just stuff off the top of my head.

Finding/viewing/sharing/joining chats

Adding users (can't find them, invites not being received, etc)

Broken messaging (messages not arriving, formatting not being handled or handled the same way, even on the same XMPP server!)

OMEMO issues (key exchange not working, or OMEMO keys getting lost or something when both clients involved were working perfectly fine the day before - Telegram's E2E doesn't break randomly and no, there isn't a MitM issue)

Clients asking for encryption, but then get mad because you're using Thunderbird's XMPP implementation and complain they don't like the encryption supported by Thunderbird.

Not being able to disable encryption on some clients (I actually rather not have encryption anymore because of all the problems)

Multi-device messaging issues (fresh chats can have messages going to one device but not the one the user is active on or history synchronisation)

Mobile notifications inconsistently working, even when using external push messaging providers (it works consistently always on Telegram, it's even more consistent with Matrix, even though Matrix has more push disadvantages)

Client crashes (I can't even remember when Telegram last crashed, but Pidgin crashed on me today and it has only a single XMPP account on it)

Clients failing to connect without useful information (Siskin no longer connects to my account and the only indication is that the "enabled" button becomes disabled, despite the account working on other clients and devices)

UI lockups (Miranda NG, Psi, Gajim, and Pidgin all tend to become unresponsive for short periods of time, probably a blocking wait being imposed in the UI for some reason?)

Common issues that every modern messaging application has, like voice and video, are barely supported overall despite being fully supported in the XMPP standards (file sending is even an issue for Thunderbird)

There is next to no failure recovery on most XMPP clients. Anything that fails on Telegram will try to recover and resend without needing the user to intervene and helpfully displays it's working on it.