Mono works great under Darwin/OSX. I develop against it at work daily (we also run Mono on Android and iOS). It's 'intended' to be cross-platform in the same way Java intends to be and I've encountered no performance problems with it on any of the UNIX platforms.
That said, PicoScope on Windows was certainly developed as a C#/.NET app using the MS Libraries. Mono is not at parity, library-wise, with .NET. I suspect that this precluded easy porting to UNIX and they had to do a hack job and ifdef out some of the features. Hence my ask If the Linux version is stripped down.