Interesting that both you and Sharon say this. I have never ever noticed anything unusual about my dog's poo when she's given greenies.I have had wonderful results with ClenzADent, (or it might be spelled CleanzADent) a seaweed extract that I sprinkle on the dogs' food once a day.
My second vet, which is not the one who started me on the product, noticed that it was working on their teeth and asked if I was brushing more (nope!).
I stay with natural chews - Greenies were not digested by my dogs and came out looking much like when they went in!