What is your affiliation with RocketChat?

I did. Also mattermost and matrix. Few years ago. Ended up with matrix instance. But it really depends on specific scale and usecase.

From what i remember - Rocketchat was based on meteor, which seemed dying at the time + it was computing hungry and clients were especially hungry. - Mattermost worked for a time but you were not able to get mobile notifications without paying for license (not sure about now). Sounds suprising but it was just problem. - Zullip too many dependencies and complicated install.

Matrix is more of a standard than anything. The first server implementation is called Matrix Synapse written in python twisted. I had it running without issues on 512mb vps for 30 people org with SQLITE. When you need bigger scale Matrix has some go and rust server implementations, you can also use postgres etc.

I guess you should start with what scale you need and what features you need and go from there.