As of now, it has very minimal testing, because the thing is literally still warm from the sausage factory - it's only actually been capable of successfully completing an upgrade, any upgrade, for about two weeks.
That's only temporary, though. The point of fedup is to write One True Upgrade Mechanism that works properly, which (we hope) will unify maintenance and testing. preupgrade used to get minimal maintenance and testing because we had ISO-based upgrades, and so long as those worked, there was less motivation to fix preupgrade - plus preupgrade was a pretty big hack job anyway. Since fedup is now the One True Way, it ought to get all the testing and maintenance focus.
So right now I wouldn't bet a penny to a dollar that fedup would work on your setup if it involves a non-standard grub config, but if you try it, and file a bug telling us exactly what color the explosion is, the chances are much higher that it'll be fixed than they would've been with preupgrade :)