Throughout my observation, there is always a caveat to auto upgrades in general. For example:
on the condition that there are no breaking changes so that security issues are automatically resolved/patched
There are known issues that some users can live with and vice versa based on the Mattermost Changelog. Hence, the reason why manual upgrade is preferred most of the time. Let us not forget that it is always a good practice to have backup before upgrading a system as well that some would overlook at times.
Unless there is a script available that can perform checks against the changelog for specific keywords for the latest version >> auto backup the instance (database, config.json, file store) >> auto upgrade the Mattermost server (basically all the steps listed in Upgrading Mattermost Server), that will be pretty awesome.