Engine stopping in a spin, plus how easy is can be to inadvertently enter a different spin mode to the intended mode.
YouTube - Broadcast Yourself.
From a student's perspective, I have a slight nervousness of spins on account of attending a fatality as initial response volunteer fire crew in which an aeroplane stalled and spun in. I was at the pre-solo stage at the time and it killed my confidence.
So, when I finally got the latter parts of my PPL training, I specifically asked to cover spins. That lesson was one of the best flights I've ever had, if nothing else it showed me how good an aeroplane a C152 is. Perhaps that's why I'm so loyal to them.
I guess what I'm trying to say is that if your students are willing and interested to try spins, do it. Teach it well, teach it safe, and I'm sure you'll turn out slightly better pilots as a result.