There's a couple of good ones I can think of without scratching my head. One is that while you're messing with that thing on the roof, it's all too easy to slip and fall. Even worse when the weather gets nasty, and as we're getting into Winter up here in the Northern regions you'll find that out soon enough.
Second, the height. Right now, my Chevy is just shy of 7 feet tall. I can get it into the Delta Sonic car wash, but only just. Better figure another two feet with that generator mounted up there. Are you going to remember that next time you go through the Burger King drive up? First time you forget, it could get expensive fast.
Third--- OMG, the incredible noise!!!! That roof panel is going to amplify the noise the generator makes.
Forth--- the roof might not be the best place to mount that hundred or so pounds either. Many vehicle makers limit roof loads to about that, spread out over the entire roof. You're concentrating that in one area.
Fifth-- rain and snow--leaks in your roof. I installed a roof vent in a Ford van many years ago, I never could plug all those leaks.
Best idea I've seen: A receiver hitch mounted on the front of the van, right under the front bumper. A carrier is fastened to this, and the generator is mounted to the carrier. Just make sure you have clearance so engine cooling isn't negatively affected. After that, you only have the problems you're going to have on any generator hook-up.
