The only anectotal experience I have with Matrix and with XMPP, there is always some message that does not immediately arrive at the recipient on mobile. Signal works just as reliable as WhatsApp in that regards.
This may be because of how push notifications work on those platforms, that is applications have to contact a centralized platform (Google/Apple) to notify users instead of relying on traditional methods.
There are standards in the XMPP ecosystem (XEPs) to do that right though it's not enabled by default on most servers yet (see prosody's mod_{muc_,}cloud_notify).
There are standards in the XMPP ecosystem (XEPs) to do that right though it's not enabled by default on most servers yet (see prosody's mod_{muc_,}cloud_notify).