The P-38 itself isn't difficult to fly, its gentle stall characteristics and no torque make it one of the more easier planes to fly.
However, fighting in a P-38 isn't as easy as some are trying to out to be. To be successful in the P-38, there are a lot of things the pilot needs to have a good knowledge of, not just how to do it but when to employ that knowledge. If a player doesn't have a good knowledge on how and when to use flaps, they're not going to have an easy time fighting in the P-38. If they don't know how to properly use throttle control, they're not going to have an easy time fighting in the P-38. If they don't know how to coordinate turns, they're not going to have an easy time fighting in the P-38.
It is also important for the P-38 flyer to have a good knowledge of both Energy and Angles fighting, when to employ each and how to switch between the two mid-fight as the successful P-38 flyers in this game are the ones that mix both Energy and Angles fighting in a fight.
There was a pilot that boasted he was able to 'pwn' 95% of the players in his Spit VIII and decided to take a few tours to learn the P-38 so he could 'pwn' the top P-38 flyers. This player quickly found out that fighting in the P-38 wasn't as easy as he had thought and ended up quitting the P-38 because he grew frustrated at not being able to fight in it and got his arse handed back to him on each flight.
To recap, is the P-38 easy to fly? Yes it is. Is the P-38 easy to fight in? No it's not.
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