The balance is the question. Honda mentioned something about "big bang" I believe just after they released the info and made me think that the rear 2 cyls could be larger than the front 3, each bank firing together much like a V-twin. When I saw the engine in the bike the design made sense, the rear 2 cyls fit snugly in the frame and the front 3 spread wider, making use of the extra space. I havent read anything since, so thats just a theory.Like any other engine, with pistons, rods, crank, fuel, and air. Cast it with 3 cylinders on 1 side and 2 on the other. The VW version is their VR6 engine with 1 less cylinder, very close to an inline engine.
Honda has made (rather successful) race bikes with true V5's. It more than likely has weird balance issues but there is no real reason a v5 couldn't work.
Ducati also used a "big bang" design. The riders said the power delivery was more controllable.