IMO, Matrix is definitely the best protocol for sending oneself messages in a running thread across all of one's devices. That is one excellent use case, and it's a real one.

It is also great for the same thing, but with other users, too. I have no idea why its current uptake is so slow among small groups. Is it the apps? The fact that you need to find or join a host (or self host)? Merely being "one more app" and "unknown"?

I wonder what the Matrix team is doing to smooth out whatever adoption issues it has. I think it is worthy of more adoption.