"The problem I'm having with Audi's LMP1 is that they're running a diesel engine and not a petrol engine. The people over at Porsche, Toyota or Nissan aren't stupid either; there's a reason why their engines run on petrol".The problem I'm having with their LMP1 is that their engine is in the front of the car and not the back. The people over at Audi, Porsche and Toyota aren't stupid either; there's a reason why that engine is in the back of the car.
See what I did there? That's another example of two different, but both valid solutions for the same problem.
People used to race on petrol engines for years and then Audi came along and put a TDI in their brand new R10 LMP car. People said that would never work. The engine produced less power so they'd be slow on the straights. Audi heard all those doubts, ignored them all and kept winning. I'm sure they'd been well aware of both the advantages and disadvantages before making the final decision to do the switch.
Same thing now with Nissan. Mid-engine RWD cars have been successfully used in racing for years, but it doesn't mean that's the best layout. There are front engine RWD GT cars and WTCC racers are FWD. All configurations have their strong and weak points.
The guys at Nissan decided to go with a front-engined FWD layout (with an option to go "4WD" using hybrid power) to move the balance more to the front and ease the pressure off the rear wheels, but I'm sure they're aware of what they give up in return (like bigger understeeriness of FWD cars).