I concur with some of what you are saying here, the F-5 was small and nimble but lacked endurance. But which F-5? The F-5E "Freedom Fighter" was a whole different aircraft than the F-5B. And the F-20 Tigershark was an export hopeful F-5 derivative that was on par with the F-16 series, some say it was better.Let me put it this way concerning combat vs. training. The US used to use Aggressor F-5s to fly against F-4 Phantoms, simulating smaller Soviet planes like the Mig-21. The Aggressor units handpicked their pilots, taking only the best of the best, usually only the F-4 pilots that could beat them. Training scenarios were set up to advantage the Aggressors and it was a rare F-4 crew that could be routinely beat them.
The F-5 was a great little plane, and the pilots that flew them loved them. Probably loved flying them better than anything else.
But taking them into combat was something else. They only had a range only radar for the lead computing gun sight and with that no ability to find targets. Minimal to no EW equipment. Little range and endurance capability. Tiny bombload capability, little guns (low rate of fire), no radar missiles, etc. Great for training, but very limited capability in the actual scenarios the US would find in combat. Because of this (besides Aggressor training), the F-5 was only used for foreign military sales. Just like the P-39 after the opening stages of the war.
No matter what it looks like on paper, or in testing, there is nothing like actual combat to establish what works and what doesn't. To think we know more now than they did during WW2 about this subject is a big mistake.
Again, the P-39/P400 was lacking, but that doesn't mean it was a bad plane. It evolved into the P-63 Kingcobra and was on par with any other main mass produced fighter of the war if only slightly interior to P-51Ds and P-47Ns. And the F-5/P-39 is actually a good analogy, both got kills in the hands of skilled pilots operating in the limitations of their warplanes!
If you do some photo research of the postwar racers the mods they did to the P-39/63 series it becomes a whole new plane!