Unity's multi-platform track record is quite solid at this point in time.
I wouldn't bet on exotic Java attempts of "write once run everywhere" though.
I wouldn't bet on exotic Java attempts of "write once run everywhere" though.
You're not allowed to do JIT on consoles or mobile and you're memory constrained. A PC-only game engine is going to be a tough sell these days.
Also, unless you're bundling a JVM (not sure about the legals here) you have to make your Windows (i.e. primary) customers install the obnoxious Oracle Java runtime.
As for Java, Notch managed to do it for Minecraft, except for the mobile port which I think I remember reading was re-written from scratch on iOS because the performance was terrible.