FWIW in my experience upgrading Android versions works, mostly, as long as you remember to uninstall the old Google Play services and then install the new ones.

However, without a tested migration path, it may break your phone and make you factory reset + reflash the ROM if it doesn't work out, and there's nobody you can turn to or blame when that goes wrong. There's no official support, but that doesn't mean it'll never work.

Testing migration paths is a massive pain, especially when you're upgrading a whole bunch of parts all at once, and volunteers have more fun and frankly more important things to work on.