The P-51D Mustang is a handful on takeoff. This if from a Flight Journal on the Mustang - ‘‘Staying centered is no overly simple task; the P-51D’s Rolls-Royce Merlin engine and Hamilton Standard four-blade propeller develop a lot of torque. Right rudder in serious quantities is required to offset the pull to the left (five degrees of right rudder are, in fact, pre-set by the pilot prior to takeoff to ease rudder pedal forces), but once the airplane’s airspeed gets to the point where the rudder and vertical tail have acquired some authority, the pilot can reduce the right rudder input and start concentrating on other things.’’
I think the problem can be multifaceted, that’s not to say the flight model is accurate you’d have to find someone with genuine P-51D experience for that. However when you’re watching a YouTube video of a pilot flying a P-51D now, they almost certainly have thousands of hours of experience, probably tail dragger experience before they would go on to fly the P-51D and therefore can make the difficult, look relatively easy with the smoothness of the take-off.
Also the rudder physics in FS2020 are, in my opinion, a little much. With most aircraft being really sensitive to rudder. Also of course as the speed increases the rudder effects change, partially on take-off and with a propeller driven aircraft as youre getting more direct airflow over it as it accelerates.