[Aerkuld]

The benefit of the flat plane crank compared to a cross plane crank is basically a lighter crankshaft. This allows quicker response from the engine as there is less mass to accelerate.
The cross plane crank has a benefit also. It's inherently better balanced, so there is less vibration.

Older engines used flat plane cranks because they are a very simple design. It wasn't that there was any difficulty with these, it's just that the engines gave a lot of vibration. The cross plane crank was a newer design, created to improve the engines balance.

Make sense?