Bravo to Signal for being easy enough for my family to use!
I tried Element. UI felt slow, I was unable to find notifications in scrollback. Clicking the notification button would take me to random messages.
Signal and XMPP (via Quickly) have a simple phone number based signup workflow that my family have grown used to.
My family are not happy on having to remember/use passwords/keys. That's a shame, but is ultimately a constraint I have to deal with when persuading them to install/use an IM app.