I setup an enterprise-grade Mattermost server in a weekend. The hardest part wasn't even Mattermost itself, but getting SMTP working reliably in the age of DKIM/SPF/DMARC policies. This could be sidestepped by using any of the well-established email providers.
No reason to use Slack when such options exist, but also no reason to write your own chat app, either.
No reason to use Slack when such options exist, but also no reason to write your own chat app, either.