I've noticed that, for me and my wife at least, #Signal isn't always reliable for time sensitive messages. Even after exempting it from the Android power saving and everything it's almost like it goes to sleep sometimes and notifications don't pop until we open it up. The #XMPP app Conversations.im however is very reliable. Running a packet capture on my phone shows that when switching networks #Conversations immediately reconnects, but Signal doesn't, sometimes for several minutes.
This also lines up with our observation that Conversations is generally more reliable for calls in weak service areas or while driving. It seems generally more reliable and for a while was the only one that wouldn't say "ringing" unless it had actually made contact with the other person's device. If it just sat on "calling" but never switched to "ringing", you knew the call didn't go thru, for one reason or another. Signal did eventually add this, but it still feels generally less reliable.
@gerowen could you Analyse ntfy.sh also?
It is an implemenation for @unifiedPush@fosstodon.org standard of push norification ...