Yes, the files work cross platform. The only issues you'd have are with difference in what fonts and plugins are installed on the two systems.

Thanks a lot.

To what extent does that include for example fi/fl in file using old Mac T1, going to Windows using old Windows T1 ? Likewise for other Win/Mac traditional encoding differences?

InDesign translates the f-ligature character values to Unicode in a document if the user typed them on the Mac or imported them, so they will work cross platform. Likewise, other differences between platform encodings are not an issue because they are always resolved to Unicode. The only cross-platform problem that I remember right now has to do with fonts that don't have the expected CMAPs.