Absolutely. I'll look for some studies now. I was just going off of what we always told clients back when I was working at an animal hospital. If I remember correctly, it had to do with ingestion of the chemicals used, especially in regards to cats and for those with small children. Kids are notorious for putting things in their mouths, and with a cat's grooming habits, well you get the picture.
We advised against them because unless an animal was supervised at all times, they could easily slip off and chew the offending collars (heck, my dogs do that with regular collars!). Anywho, I'll try to find some references.