All of which indicates that poor instructor training was the most probable cause...
I've read a lot of total and utter nonsense written about the Firefly - including 'The (US) Air Force didn't practise glide stalls' and 'It wouldn't recover by releasing the controls'.....
Any aeroplane will kill idiots; sadly, the USAF seems to have proved it.
Of course for the ambulance-chasing bloodsuckers, it's easier to allege that the aeroplane was at fault, rather than inadequate training, mishandling and late take over of control by the instructor.
I didn't like the T-67A due to its abysmal roll rate and lack of V-P prop, which made aerobatics a pain compared to the Bulldog. But I had absolutely no qualms about the spinning behaviour, provided that the fuel loading was carefully observed and the correct recovery technique used.