Heartgard is relatively safe for heartworms, but the disease can progress for the two of so years that the treatment is going on. And there are risks for those two years. A lot of rescues do it this way, but then when the dog is placed, other vets will switch treatment. It just isn't the standard of care for most cases... I think something like milbemycin could cause a big problem though.
Testing is done yearly here or every other year if a waiver is signed.
But then, I've seen much worse than failure to prevent or test for heartworms, so I guess vet med has much bigger problems, unfortunately.
How about failure to vaccinate for rabies but expecting the vet staff to handle your animal with care (with no medical waiver)?
It puts everybody at risk.