AI DMs are just dodgy in general, player controlled MiGs are easier to down, providing the server is not messing up and many of your shots are going phantom due to netcode loss.
Though the MiG-15 being too strong may have a point considering the MiG has 2 unprotected, non-self-sealing fuel tanks, it should be very vulnerable to .50 cal API below 25,000ft. Above 25,000ft where most of the combat in Korea took place, there was not enough oxygen for the magnesium to burn, but say on the Burning Skies server, where combat is basically at sea level due to mission design, the MiG should basically act like a Zero when its hit in the fuel tanks.