The protocol and the service behind it are state-of-the-art, but it's a tough sell if you're coming from something that just works on every device, like iMessage or WhatsApp.
I do receive spam in Signal, because i had to register a phone number.
I loose my chat history if I do not log into the desktop client for FIXNUM days.
The desktop client may crash as soon as you kill its supporting terminal.
I have tried the user name feature once and signal reported, that they had lost my username, I would need to create a new one.
I have not tried backup and restore. So far I am not in the mood for a potential failure.
This thread was depressing to me — I can't believe we're still dealing with the lack of a truly open near universally used secure messaging system.
I bridge signal to matrix on my homeserver using signal-mautrix: https://github.com/mautrix/signal
This allows me to use different phones without going through transfer/wipe. Still needs a primary device though, which was the iPhone until yesterday.