I'll start with the F-84 since who could possibly have an opinion about that one?
Personally, I'd say the F-84 was a flop for the following reasons
The air-to-ground capability was its saving grace, because of it's limited air-to-air capability. I should point out that the US Navy's F2H & F9F were both able to be used in air-to-ground missions, and were probably better designs overall (the F2H was also used as a nuclear strike plane, and it was more agile).
Personally, I'd say the F-84 was a flop for the following reasons
- The F-84 was an interceptor (though the initial origins of the F-84 was a jet-powered P-47, the idea was ultimately done away with because the centrifugal flow engines of the time would not fit in the P-47's fuselage): The primary role of interceptors is to shoot-down bombers (though the ability to shoot down fighters at the time was assumed), and this was achieved with a rapid rate of climb, a rapid acceleration, a high top speed and altitude, and heavy armament (i.e. cannon); while range is desirable if you can get away with it, it's not the overarching goal.
- The wings were thicker than were probably prudent due to the desire to carry a large load of fuel: This thickness caused problems at transonic speeds (most all wings did to some extent, but as a rule, thinner was better than thick)
- Problems with the strength of the air-frame lead to increase in weight that resulted in a heavier than ideal wing-loading: It could not turn with the F-80, nor could it climb as well due to the weight
- The design was almost cancelled because it could not meet any of the requirements for which it was originally built
The air-to-ground capability was its saving grace, because of it's limited air-to-air capability. I should point out that the US Navy's F2H & F9F were both able to be used in air-to-ground missions, and were probably better designs overall (the F2H was also used as a nuclear strike plane, and it was more agile).