The problems with data models can happen at small scale. I remember the first large-ish project that I built, it had a few different components but the problem started when I tried to introduce data models to the Java and Python parts (I had it in my head after reading a book that I needed domain objects or some other nonsense in each language)...mistake, the data was still changing, it took forever to make changes, wasn't critical but I learned my lesson very quickly.

One perspective on this is that many ORM libraries don't take the DB as the source of truth (one very good ORM library that does, and which saved me in this case, was JOOQ). I think a lot of small-scale problems could be solved this way, monorepo is just another variation of this solution: moving the source of truth into the repo.

It is surprising to me how often variations of this problem come up. Obviously, there are solutions from multiple directions: having cross-language definitions (ProtoBuf), Arrow (zero-copy abstractions suitable for high performance), maybe even Swagger which comes at the problem from documentation...but I think this problem still comes up anywhere (and the DB approach is, imo, a very strong approach with a decent ORM at smaller scale).