As for the P-39, which I know is a zombie horse (and who isn't sick of zombies these days) - it was a flawed design which also had significant potential. It was basically hard to learn to fly and hard to learn to optimize in terms of maintenance and field modifications (esp. vis a vis thing like the balance with the big gun and ammunition), which is something that all WW2 fighters contended with to some extent or another. The Russians made it work, partly because their Theater was ideal (where low altitude, short range fighters were the norm) it suited their fighting style (maintaining speed, shooting from short range) they did a long workup on it before throwing it into the breach (very different from Allied units in the Pacific), and perhaps most important of all, Russian pilots who flew it had already cut their teeth (and been through a brutal winnowing process) flying in combat with planes like I-16s, MiG 3s, LaGG-3s. If you lived through a year fighting the Luftwaffe with one of those, a P-39 was like a dream, both in terms of flight characteristics and performance. I.e. the I-16 etc. were all known to be 'twitchy' and easy to spin, and with difficult spin recovery traits. So if you were a pretty good pilot in one of those a P-39 would theoretically not be so much of a challenge, and then you can take advantage of the nice rate of climb, speed, streamlining (which helps keep speed up) armament etc.
The P-39 also had working radios, and I also believe it may have flown better in cooler or cold weather. They certainly had less problems winterizing them than they did with Hurricanes, Spitfires or P-40s.
It is and will always be a mystery, how the Russians made it work (and unlike some of our Luftwaffe fans, I believe they did) while nobody else could, but no more of a mystery than how the Finns did so well with the F2A compared to the USMC, and I think it boils down to many of the same factors.