This is really the crux of the matter. If us Americans already had the war-winning fighter in hand in Dec 1941, why did we bother with any other designs?
The answer is: the USAAF at the time, meaning its generals, its procurement staff, and its pilots, all believed that better aircraft could be had.
Why did the Brits reject it? They believed better aircraft were already on hand.
The folks who operated it replaced it when they could with better aircraft. That says more than any data sheet ever can about the plane's utility under operational conditions. Compare and contrast that to, say, the A-10, which has survived numerous attempts to replace it. The folks at the pointy end of the spear know a good thing when they see it. They also know a problem-child when they see it.
All the gainsaying from datasheets don't mean s**t from shinola when it comes to the warriors who fly, fight, or benefit from the plane in question. They are the only experts we can really regard, and their expert opinion was that the P-39 should be replaced where possible, and shipped off to the USSR.
They knew the airplane, then, flying and fighting it, better than any self-proclaimed "expert" online.