It sounds like, if Matrix worked seamlessly (which is not the experience of... anyone(?)... in this recent thread https://news.ycombinator.com/item?id=44617309 ) it would give us federation and better identity access management (compared to the Mattermost community edition anyway) which would be really nice
It also has video calls built in... also really nice, but people are saying since they migrated from jitsi to their MatrixRTC, setup & admin have been a nightmare.
Element X seems to be the more popular client, though it sounds like, while faster than Element, many features (such as threading) are not yet available, and people have been lamenting the extreme sluggishness of Element