My own feeling is that I've read a number of stories of experienced instructors discovering unrecoverable spin modes in aircraft like the Cessnas.
And that's what they were - stories.
Sec. 23.221
Spinning.
[(a) Normal category airplanes. A single-engine, normal category airplane must be able to recover from a one-turn spin or a three-second spin, whichever takes longer, in not more than one additional turn after initiation of the first control action for recovery, .......
(iii) It must be impossible to obtain unrecoverable spins with any use of the flight or engine power controls either at the entry into or during the spin; ..........
My abridging, and bold.
If any pilot managed to create an unrecoverable spin in a spin approved Cessna, they were outside the limitations, or their skill. In a proper training environment, with skilled mentoring, lots of altitude, and proper preparation, training spins are perfectly safe in 150/152/172s.