Koolhoven FK 58

Universally considered to be one of the shittiest fighters of early WW2, but there seems to be very little actual information about it somehow.

Whatever data there is shows it to be roughly similar to ms 406, Bloch MB 15x, etc, if not somewhat better. Supposedly didn't meet it's performance specifications (then again, so didn't 406...). Production was a kafkaesque clusterfuck. Only flew with jerryrigged RCMGs and didn't see any actual combat, so nothing to go by there either.

Tl;dr: how bad did it really suck, and could something actually useful have come out of the project eventually?

It was intended as a 'cheap and cheerful' colonial fighter, and something that could be produced outside of France because of the stresses/strains re-armament was putting on French industry.

As far as the actual design goes, it was a compact and neat little aircraft but it was outdated in terms of structure and materials. It was of mixed steel tubing and wood construction, with a metal forward section and fabric aft of the cockpit. The wings were partially wood, partially metal, with the innner section skinned in metal and the outer section in Bakelite.

There are a few accounts (French and Polish) that it was a pleasant aircraft to fly and that it was surprisingly quick to climb to medium altitude. However, production aircraft weren't very quick (max speed is quoted as somewhere between 450 and 500 kph, typically about 475-485) and the poor situation with French supplied equipment meant that it was temperamental in service.

One French account suggests that pilots considered it a better fighter than the MS 406.

The French wanted to deploy it to French Indochina and West Africa. I suspect that if it had gone up against the Japanese, it would have gotten rather chewed up by the A6M and Ki 43 combination. It may have fared a better in Africa against the Italian biplanes and G.50 and C.200 combination.