> Have you ever tried a complex branching model like gitflow with multiple teams, and hoped for everyone to be on the same page?

My company does exactly this and I am happy to report there have been no issues. I think the key reason we don't have issues is we are strict about maintaining backwards compatibility between services and also try to maintain forwards compatibility where possible.

> “What’s in production” becomes an existential question, if you’re not careful.

We solved this by embedding the SHA's of a build's constituent repos in the build artifact.