While I agree with your conclusion I don’t think reliability is the best reason to switch. Signal is probably less available than FB services and many popular Matrix servers have semi-regular downtime as well.

Although in favour of this argument:

• Setting up a “backup” service for when FB is down is a nice way to gently ease people onto something better.
• Matrix or XMPP does allow you to be in control of the availability, (although to be honest you will probably struggle to better than FB Messenger or WhatsApp).