No one ever addresses why IV pills is worse than the same roa for heroin. The cuts used in heroin are not exactly good for the veins.
The worst problem I see is the talc, in pills that contain it, as its insoluble and those super-fine particles end up in tiny veins in the lungs blocking proper blood flow and thus oxygenation of the blood.
Don't forget that there are plenty of insolubles found in heroin, too, though- just probably not as bad as talc.
I was going to write that the mg-active to mg-total ratio favors heroin, but that's pretty variable. I don't think it's true for the tiny mallinckrodt 30mg oxys (the little blue M 30 ones), since they're a little over 100mg total, so a bit less than 1:3. Heroin is all over the map, anywhere from 1:20 to 7:8 if you're lucky.