Compared to a conventional plane, the mid engine layout is likely to weigh more. You have the transfer gearboxes, driveshaft and an extra firewall. The original P-39 prototype had a conventional taildragger undercarriage, but it had problems getting the tail up on takeoff so they went with a tricycle layout which added yet more weight.
Having most of the guns in the nose caused a rearward centre of gravity shift as the ammo was used up. Another issue is where you put the fuel if the engine is occupying the centre of gravity, the P-39 and P-63 both suffered from small fuel tanks.
The German, French and Soviets all had inline engines that could accept a cannon firing through the propeller shaft, taking away one of the main reasons for the P-39 layout.