Basically any Luxeon V light might have a donut hole in the beam. It's not something specific to Surefire, and certainly not the U2 specifically. The problem is that Luxeon V LEDs are really four diodes placed closely together, but they're not always quite close enough and as a result you can get a dark spot in the middle.
Just as with tint, donut holes fall under the "Luxeon lottery". Whenever you buy a Luxeon, you'll probably get a good tint, but there's a distinct possibility you'll get a bad tint. Whenever you buy a Luxeon V (like the U2), you probably won't get a bad donut hole, but there's a distinct possibility that you will. It makes not difference what model Surefire you get, all U2s are different and all L2s are different. There's nothing Surefire can do to fix the donut hole problem, or tint problems, except maybe start rejecting some of the worse LEDs that they are currently putting in their lights, but I doubt they would do this as it would cut into profits. You just have to hope you don't get a donut hole and/or and terrible tint when you buy Luxeon V lights.
Personally, I don't think it's a good reason not to buy Luxeon lights, but some people are really bothered by "off" tints or donut holes.