We can't talk about #RocketChat, #Mattermost or #Slack without mentioning two sore spots:
* Desktop clients
* Mobile push notifications
It may have changed, but the majority of these desktop clients (and a few mobile!) are Electron/Chrome wrappers. They don't look right in any given OS. Not a huge deal, but definitely not helpful for folks that like to chat without exposing themselves to hacks or tracking, ie using a web browser.