Except that it pumps it through your brain and lungs. Take tren for example. If you just nick a vein you can cough like you're dying. It's happened to me. It can be minor or very scary. That's just a nick. If you were to push 100mg of tren directly into a vein and that hits the lungs who knows what that would do. Who knows what a concentrated amount of dbol inject or Winny or whatever would to if it passed through the brain or kidneys etc...
Not just taking the heart into account but the brain and organs as well.
Also, I remember reading that it takes more air than most people realize to stop the heart. It's like 5+ cc's. So those little air bubbles in the syringe aren't going to hurt anything. I don't even concern myself with that.