According to their compatability section (see: https://ruffle.rs/ then scroll down), it looks like, after years of work, that they're only up to supporting 10% of the AS3 language, and only 5% of the AS3-based API. Yeah, I don't think I'm going to be regretting all those expensive Flex development books that I recycled a while back.

Flash/Flex: it's not coming back, just let it fade away?