The Mattermost API is a superset of Slack's. "Most" integrations "just work", the rest need a minimum of tweaking to DTRT.
Mattermost API being a superset of Slack's seems to be mostly true, but looking at the features our Slack app uses I don't see block kit (https://api.slack.com/block-kit) or message scheduling. If we were to integrate with Mattermost it doesn't look like it'd be a ton of work though
Have to say Mattermost's API docs are so much easier to read than Slack's.
Slack's API documentation and handling of accounts/bots is so obscure. I've never seen anything that bad from a major company.