C# interoperability loosens guarantees (particularly immutability) that F# code normally relies on. There are surprising limits that come up in generics because of how they map to C#.
The larger issue is the C# community loves OOP so you often have to wrap these libraries into something more “FP” if that’s how you want to work.
Overall it’s far better than having nothing (looking at Haskell, OCaml as much as I enjoy them!)