I really hope the UX improves considerably and basic features actually function as expected. Notifications for new messages on OS X were broken for several months last year, for example, and notifications on Android are still hit-or-miss (literally "miss" when you miss a message for several hours!).
I haven't really had any issues per se, at height about a year ago I had just over a dozen contacts on signal. The biggest UX deal breaker was no profile pictures. You can set one, but almost no one will ever see it since almost everyone uses Gmail or another cloudy email provider that will sync or store ancient Google+ (or equivalent) avatars to your contacts. Signal chooses to display a contact photo from your device over what a user sets. This is insanely counterintuitive approach and no other app works that way, meaning no one expects it to work that way. My family and all but two contacts moved back to WhatsApp. I'm glad to read they're focusing on UX now, but I can't bring up switching again with these people. Maybe when or if phone numbers are abandoned, then I won't have to link/associate contacts to my address book and see those old avatars.
