My dog once got very sick from the pig ears or maybe it was rawhide chews-- I quit buying them, its only dog biscuits from the US now. And I've heard the greenies can lay in their stomachs.
Well, I dont know about the gland flavoring, but did want to comment about the dog items. There have been far too many issues with China produced dog treats that I am also only feeding US treats (and check carefully, some of them are packaged in USA but the actual food product comes from China!).
On the Greenies, that is no longer true (I looked them up a while back). Apparently the original Greenies could build up in the stomach, but they changed the formula quite some time ago and they are now 100% digestible and are not supposed to get stuck anywhere anymore. Though my dogs eat them so quickly I dont see how they can help the teeth much, but they do have better breath for a day or so afterwards.