My dogs love them too and have had no problems with them. But I was at Petco not long ago and one of the clerks told me something about this. I thought he said smaller dogs have difficulty digesting them, but I can't remember. My complaint is that they're too expensive.
Nope, huge problems with Greenies--the bits end up cementing together in the gut and causing blockages--my vet alone has done 9 surgeries to remove greenie bits from dogs (including his own dog who had been eating them for years)
My vet has also had patients that have had problems with Greenies. She says she does not recommend them AT ALL for dogs - too dangerous. She said that she's never heard of a cat having problems with the Greenies cat treats though.