From my experience with Java 9/10 they've really screwed up reflection. EnumHelper, to give an example, probably won't work in Java 10 because Oracle has decided they're going to be very much more strict about illegal reflective operations. (Changing final variables especially, but I imagine this will carry over into enums as a security 'feature'). And afaik there isn't any amount of elevated privilege that allows you to bypass this, but I could be wrong.
Is the plan to switch FML to Java 9/10? If not what's the plan to handle these sorts of things?