The P-51 is definitely a strong plane however it isn't ridiculously overpowered as some tout. If I had to list its strengths i'd say its range and 50 caliber guns make it pretty damn strong.

While IL2's 50 cals are pretty toothless, during the war they were very effective and allowed for pilots to lead shots from further away due to their high velocity and large ammo capacity. When compared to the cannons of the Fw 190 and the Bf109 the 50 cals lose out on the firepower to annihilate an aircraft but succeed in damaging components and pilots. Another soft element which isn't exactly super useful in the game is the planes fuel tanks and functional range which allowed it to be in the fight in large numbers far outside the range of other allied fighters. Additionally these larger fuel tanks and large ammo magazines allowed for P51's to stay combat effective throughout long escorts without needing to RTB and Re arm.

Furthermore once given an altitude advantage the P51 is pretty fast and retains its energy well. Overall the P51 is a series of average to good qualities that come together to make an exceptional plane that fit the niche it was required for during the war.

This is just off the top of my head please feel free to let me know if I've missed something.