Mattermost is great, but it's not decentralised, it doesn't federate, it's not end-to-end encrypted, it's not based on an open standard, it's vendor-locked to Mattermost, only has one usable client implementation, and is rather aggressively open core (unlike the BundesMessenger distribution which is entirely apache-licensed FOSS). I'm also not sure that whether deployments easily scale up to million+ users like a big Matrix deployment can.

It's worth noting that if Mattermost adopted Matrix, like Rocket.Chat has[1][2], the vast majority of these limitations would fall away :)

[1] https://www.rocket.chat/press-releases/rocket-chat-leverages...

[2] https://matrix.org/blog/2022/05/30/welcoming-rocket-chat-to-...