1a) e2e is a choice that affects experience in riot, a lot - you can't use search, you don't see recent images/files ...
1b) you have to verify each others devices, N:M; although they're working on a change
2) I hoped that riotX will be a lot better at the end of this year, but there're still lots of bugs, messages get stuck at the bottom, mobile - mobile e2e can't be decrypted sometimes, you have to have another device online; and you can't even zoom in or save an image that's sent to you which is infuriating
3) matrix.org homeserver performance, although a lot better now is still sometimes utterly slow
4) riot is still missing crucial functions like copy entire message, online status, jump to date, and lacking any good notification system(the current one is really simple/stupid) and you can't even enable notifications without sound e.g.
4) also it's still lacking custom emote support although many ppl asked, there's no option to pin/save messages...