I have heard that the team that put it together actually tried to hide that fact from the company (for the glory, I guess). But that could be apocryphal.
Mattermost didn't work out of the box, and certainly not the way and at the scale Uber needed it to. I'm not overly familiar with the technical details, but one thing in particular stands out as an example. There was a Town Hall channel that every user had to be a member of. This unfortunately did not scale, and not enough ACLs were available to limit all the ways users could use this universal room. Eventually they really fixed the problem, but it was a tremendous pain point for a long time. There were a lot of fundamentally "less than great" things about Mattermost that had to get updated to work for Uber.
There was the amusing time employees found out anybody could change the topic in the room, even if our chat permissions had been disabled. It was absolute chaos for at least an hour, I can't remember if it actually negatively impacted the deployment though it sounds vaguely familiar.
It's pretty telling of employees that badmouth the uChat team. That team ultimately was trying to do what they thought was best for the company, even if at the time it seemed like they bit off more than they could chew. There was no other engineering team so directly visible and exposed to the entire company internally like they were. People dismissive of their efforts are generally not used to the difficulty of making so many very vocal customers happy all at the same time, and could be more sympathetic.
https://news.ycombinator.com/item?id=19101617
The fact that uChat is commonly mentioned by Uber employees as being the "custom chat solution we built in-house". It leads me to believe that comment that the team tried to hide that it was built on open source.
I don't doubt for a second that scaling Mattermost for a huge organization like Uber was a big undertaking. But it seems disingenuous for people to always mention that Uber built uChat when it should be more like "Uber put a lot of work into Mattermost to scale it up."
To be honest, I don't much like Slack because I feel the desktop app doesn't feel like a real macOS app. And I don't use all these features. So in the end, IRC would be fine for me. But it wouldn't for the rest of the company.