I have a lot of gripes with it, but since I only use it with a few people (if I try to switch someone to an unfamiliar platform anyway, I'd rather go with XMPP), my biggest one is not allowing desktop registration officially. I had to use signal-cli to register, and then it kept giving errors for trying to bind the official client. Maybe it's for the best since the client is Electron... But still, very disrespectful to people who refuse to use smartphones. Such an arbitrary requirement, they have phone numbers already...