When OP says they built their own internal chat app, that's not entirely accurate - uChat is built upon Mattermost.
https://eng.uber.com/uchat/
Mattermost has been rock solid for the last couple of years for my team BTW.
When OP says they built their own internal chat app, that's not entirely accurate - uChat is built upon Mattermost.
https://eng.uber.com/uchat/