I would imagine that the pilots who ‘liked’ the P39 had not flown any other operational aircraft at that time.It was used more as an advanced trainer by the USA than as a bone fide combat aircraft,although i know it was used a bit by them in one theatre.In Tex Johnston’s autobiography he quite liked the P39(he was a test pilot for Bell Aircraft at the time) although he seemed to tentatively admit that without ammunition loaded(or after it had been fired??) the aircraft might ‘tumble’ during combat manoevering.Presumably very sensitive to c of g position then!!
He raced one to victory in the thompson trophy post war but really the aircraft was not a good design for combat ie over complex,poor pilot visibility and extremely difficult to bail out of(to put it politely)