FWIW, I didn't keep up with Matrix to well in recent times, but
I remember having a look at it back when it made its first
splash (the LWN article at all) and at the time I considered
it the most promising-looking IRC replacement attempt yet.
It had some of the traits of other attempts that failed
before (larger per-message overhead, lots of complicated state
on the server, problems with scaling to really large channels)
but also made a lot of tasteful and practical choices.
I understand they changed their public focus to portraying
itself as a bridging solution to create any adoption, but at
the core is a good chat system of its own.