Mattermost

El chat, es muy fácil la comunicación interna entre departamentos lo que me facilita presupuestar diferentes partidas antes de dar precio al cliente

De momento nada, estamos valorando las opciones

The interface is pretty much self-explanatory. This is why the user acceptance is quite good. Customizing the interface is straight forward and can be done at user level.
The open source cummunity around Mattermost is really nice and welcoming.
Apps for every operating systems are available and easy to use once manually set up.
Scalability was never a problem with 1000+ users.

Setting up integrations is more complex than with Slack, where integrations can be installed with one click in the backend. There is an app directory, but the apps need to be set up manually. Once set up, they just work. Incoming webhooks are the only exception; they work out of the box.
Mobile perfomance is gradually getting better, but it's not yet up to Slack's par.
The Windows-app is not yet ready for big scale rollouts via group policies.

Mattermost is constantly being developed and you can tell that it's going further. In addition, as a collaboration tool, it provides the most important information. No more and no less.

At the beginning (approx. 2018) it was still a bit unstable and some important features were still missing. This has gotten much better in 2022. Over the years, with many users, maintenance and upkeep should never be neglected. Otherwise it will be slow and have long loading times when changing channels etc.
Plugins work, but not all are mature (e.g. Jitsi caused problems for us)