Uptown: Basically complaints come down to relative turn performance, according to tests the P-51 turned better than the Jug, Typhoon, and Tempest. It does not in game. I personally find the Jug just plain handles nicer and can follow things like a Typh, 109 G-14/K, or a Pony itself around in turns long enough to get the kill more easily, which is the opposite result than pilot opinion from WWII would lead us to expect. In particular, the P-47D-11 turns noticeably better than the Mustang.
Unfortunately, there are those on the internet who don't know any better and think the Mustang is a super-plane that ought to out-turn the Zekes and the like. This tends to muddy the water and make legitimate quibbles about the modeling of the P-51 in a particular sim difficult to bring up without a certain amount of guilt by association.