randomly telling me that my connection is not secure and as a result randomly hiding messages. and then, if I try to fix it somehow, it will tell me it's nearly impossible or that they'll delete my whole history. the security model is hostile towards user.

when coming back to a community after a long period of time i have to follow a trail of links to rooms that say they have been upgraded and the current one abandoned until i reach the current one. thats stupid, confusing and time wasting

for me, I'd say: occasional cryptic error messages that prevent conversation, even using encrypted DM across multiple devices