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In my experience that isn't quite true. You usually OO for IO/interop if a C# library is being used, then its module code for the most part all the way down (e.g. ASP NET Core define a class for the controller, then have it interop to F# FP code for the most parts). With some newer F# frameworks you don't even have to do that these days.

Having some experience with large scale F# codebases its rare you define a class compared to records, unions and functions. 100's of functions, 50-100 small types, 1-2 classes approx is usually the ratio I've seen for a typical microservice (YMMV).