At risk of saying something utterly predictable (and thus, unnecessary) - and indeed, a sibling comment has already made such a prediction:
* It really seems like matrix is superior in every way to both of these.
It is much easier to backup, restore, and change devices (one of the chief complaints about both Signal and WhatsApp of course), has more cognizable (and yet less intrusive!) information displayed about the cryptological situation for any given chat, and is much more flexible. Also, it has clients which are just as stable (at least that's my experience with Element on both linux and android).
My only complaint about the current generation of Element clients is that there is, unless I'm missing something, no way to globally search across all saved chats. Which is really a blocker sometimes when using it for work.
But yeah, at the risk of sounding like I'm blinding emitting the cliche response of "why no my favorite app?!", I really think it's time to ask why we're always using and recommending signal rather than matrix.