- matrix uses a security protocol that sometimes has an absolute meltdown, leading to people not being able to see or send certain messages once the encryption breaks. Good for secure messaging (because messages stay encrypted), but it is frustrating to deal with, and will turn people away (including me).
- element doss not support multiple accounts and has no plan to do so. I hear that other apps may support it, but I'm not sure what the current status of that is.
Messaging particularly benefits from a smooth UX and I think matrix has accepted certain trade offs for the aspects it's strong in.