Aside from the lack of E2EE, why is Telegram not trustworthy?

A lot of privacy guides suggest avoiding Telegram. I understand that in its default mode there's no E2EE (and no E2EE for groups at all). If people I know don't wanttko use Signal, isn't Telegram the lesser evil given it's nicer privacy policy (than other popular ones)?

Say I use the FOSS version of it.