Why do you think it's NOT safe to spin a Puchacz?

As of January 2004 Cindy B. had identified 23 fatal spins or over 1/8 of the fleet. A high proportion of these were fatal and several involved 2 CFIGs at the controls. A few searches will yield a couple more since then.

And do you think it IS safe to spin a K21?

To my knowledge K21s are permitted for spinning. Don't they have a tail ballast box for that purpose? I believe something over 1000 have been built. I don't think 125+ have plowed into the ground.