[coolius]

For me the issue that prevented me from really using matrix is that none of the big clients support multiple servers. As a non enterprise user, this has prevented me from seriously adopting it several times.

[tionis]

Fluffychat does and I would argue is one of the big ones. Its however quite mobile optimized (basically more like WhatsApp, less like slack).

[Arathorn]

Element X finally has this on Android (in Labs) now. Web & iOS will follow.