My CPL instructor was flying for EFT when the only UK military training Firefly crash happened. The instructor in that one admitted taking over too late when the student made the wrong recovery action (which worsens the spin in some way in the Firefly). He thinks it would have recovered without a problem, but standing orders to leave the aircraft at 3000 feet if still spinning, so they did.
Nothing wrong with the aircraft. Like any with decent performance it can bite you, and it was mishandled.
Guppy
The 172 is a docile machine to the point of tedium. Its handling is horrible, but safe, so it is no surprise there were no deaths. The surprise is that anyone learnt anything from it! A terrible training machine, worse than the 152, for anything but a future commercial pilot flying on instruments.
The T67 is an aerobatic aircraft. It was also possibly not well suited to flying over high terrain and with air conditioning (UK never fitted AC), so perhaps the procurement was not correct, but there was nothing fundamentally wrong with the design. Several military schools have had great success, and it is a popular aircraft at civvy clubs for increasing skills.