I used to have one that was an assisted crunch, and most of my clients liked it. I consider it safe for just about everybody even though I have long ago gotten rid of it. One thing I remember about them is that they were not holding up all too well.
There are better ways to work the core but I don’t think you’ll hurt anybody with it.
You would be correct in not recommending it – I’m surprised to hear that they still sell them. You are talking about the piece with the head rest that assists you to do crunches, & not the ab (roll out) wheel, right?