The reasons for that are several. My mini Mustang was a squirrelly little bird anytime I strayed any real distance from center of the flight regime. I've read others who have had better experiences, but they might be more content dwell closer to the heart of the envelop than I am. Alternatively, it could be a quality control thing, as plenty of this old Ultra Micro design is prone to failure or misalignment.
Part of the problem stems from first generation Horizon Hobby UM construction, which was and often still is, unacceptably weak in key areas of the build. The vertical tail of the Mustang is very flimsy, as is the horizontal stabilizer. This is likely a source of inconsistency as well as substandard flight handling.
The silly-cheap under cambered airfoil is not well suited to the warbird theme of the plane, causing the pitch axis to become too unpredictable across a wide airspeed range. Air can billow asymmetrically in the pocket underneath at low speeds, resulting in some nasty stall behavior, while the wing can tuck at higher speeds as the lack of a true airfoil snags a variety of stability perturbations.
Additionally brushed can motor is unreliable over time, I've replaced many for various issues from internal failure to wires separating. The usual HH PCB micro servos are weak, slow, and unreliable with regular use, and the tail control horns tended to tear at the ultra-thin foam hinges as a result of normal wear and tear. The plane, like so many micros in its class, exhibits a definite half-life of decay whether you baby it or not..
Lastly, the P-51 design itself looks a lot better than it feels when out of its element. This was a plane designed for extreme efficiency and power at high altitudes and airspeeds. It is beautiful to look at, but it was never designed to be a pleasant low-speed ride. Quite the contrary, the WWII Mustang was always an aptly named, ornery steed in need of experienced taming during administrative flying.
It all adds up to a price that's way too high for the opportunity cost. My overall grade is: D+