I have a large mixed language (obj-c, swift & c++) project, it builds and runs fine on all platforms
"Build for previews" also seems to complete fine.
Opening preview of a particular swift file fails, e.g.: 
Diagnostics gives:
Failed to launch swiftc.
==================================
| HumanReadableNSError: NSInvalidArgumentException
|
| com.apple.dt.PreviewsFoundation.ExceptionError (0):
| ==NSLocalizedFailureReason: too many arguments (4670) -- limit is 4096
I have managed to get previews to work previously. I switched to using prebuilt xcframeworks in the hope of improving the preview build performance, but has gone from slow to not working at all!