The problem is that with instantaneous, bonus action healing you can do at a range, you don't get a second chance. The enemy drops the character to 0 hp. Even if it has multiattack, it causes two death save failures with a coup de grace. Then before your enemy can act again, your hero has been healed, is back up and attacking as if nothing had happened.If you see a downed foe pop up due to healing once, you don't need to be smart at all to realize that finishing him off the next time he goes down might be a good idea.
It's the death saves and Healing Word that keep characters from dying.