I have sideslipped Cessnas for a couple of thousand hours and not fallen out the Sky. My Hawk XP recommend not slipping with a lot of flap out as it can give unnerving oscillations on the controls but other than that is perfectly safe.
I would suspect your Instructor sounds like he needs further training himself if he is not able to demonstrate it properly. Send him my way and I will have a go at showing him that a 172 will slide slip very smoothly and gracefully!!