Inverted 4 cycles are the most difficult and dangerous to start. The fuel can collect at the bottom of the cyclinder near the plug and cause a hydraulic lock. That is why full sized radials are always pulled through at least 4 times to make sure oil or gas hasn't collected in the inverted cylinders. Suppose the cylinder is flooded as the piston starts into the compression stroke? Both valves are closed and when you hit it with an electric starter, POW one bent connecting rod or busted wrist pin.

When right side up, a flood is easily cleared by turning the engine backwards.