Serializing file handles doesn't work, but in our experience, programs rarely run into constructs where this becomes a problem, and when it happens, there are mitigation measures that are usually easy to implement (small restructure of the program, capturing resource metadata to reconstruct them later, etc...).

We have a few features on the roadmap to help mitigate the security implications as well, including allowing users to store their program state in a S3 bucket that they own. Our scheduler can operate with only metadata about the program, so splitting the two can be an effective model to mitigate those risks.