> For contrast, I know many teams with a backend in Python and a frontent in React/JS, and I've worked on some teams with that setup. In my opinion it's virtually all downside compared to a backend in TS, as Python and TS are quite similar from a web programming perspective (I'm not referring to Python's advantage in data science tooling), so all it basically is is a barrier to your front end folks being able to make small enhancements on the backend quickly.

How big of a problem is this?

At the end of the day all that really matters is that the external representations that get handed off are the same and that everyone makes the same assumptions. Given the same set of fields manipulating a POJO with Java streams is not all that different from using modern Javascript on a blob.