Mattermost is open core. So I guess that means mattermost has lots of paid features thay relay will have to reimplement. And I wonder if those will be available in the selfhosted version. https://about.mattermost.com/pricing/
Relay is built on Mattermost team edition so it has all the team edition features. It plans to add new features as per user feedback which will be contributed upstream to Mattermost.