As far as I've seen, Ruffle never even makes the call out to the server... so at this point I don't think it's a serialization issue although some of what's in that list could potentially cause problems. The Ruffle compatability docs still say that NetConnection has 90% coverage... except for the .connect() call itself, which kinda makes me wonder why bother covering it at all?
https://ruffle.rs/compatibility/avm2