I am currently running Rocket Chat through its paces to see if it can replace Riot for my family and friends. I am tinkering with the custom emoji support and they work fine on desktop. But they dont showup at all on mobile (at least ios).
I have tried the Appstore version of the app as well as the Testflight Experimental version. any time a custom emoji is used it just shows up as a blank space in the chat.