They're pretty bad. You can't specify where the backup goes, so if you are running low on storage space (eg if you have a lot of photos or videos to back up) and add an SD card, tough luck because you can't save there. The best you can do is manually export your media (also without any choice over where it goes) and then manually move it to the SD card to make space on your internal storage. They say this is for security but if an attacker is in a position to export your backup, they are already in your signal account.

Same story with the PIN signal requires if you haven't used it in a few hours. It's the same as your phone PIN and there isn't anywhere you can change it, so it's just security theater.