Or maybe "supply issues and union troubles" could mean just that. With the brand new empire, there may have been suppliers who weren't all that enthused about the idea, and perhaps there were many union troubles. Call it "Galactic Empire Growing Pains". Just because it wasn't finished for 20 years doesn't mean it took 20 years to build. There could have been exhaustive strikes and the like, which Vader "took care of" during those 20 years. Who knows?

I know EU is canon, but I haven't read any of them, so frankly, they're not my canon.