The one thing I will say is that if the pilots are even, the P-51D is outmatched by most ENY 5 planes in at least one or two categories (At low-altitudes, where most of this game occurs, unfortunately). Also, the fact that it's directional stability is so poor makes it a more difficult aircraft to fly initially, which may contribute to one's perception of an unstable gun platform.
However, in the hands of a master, it is a very deadly aircraft.
I fly the Bravo over the Delta as well Bar, however I'm not anywhere near what I would call a "master." Fights with Spitfires still put me in a bad mood...