One of the single most frustrating things I'm experiencing in the game right now is the tendency of dwellers to stand right in front of each other in combat. While working they all retain a nice distance from each other, but as soon as combat starts several of them will immediately run to stand right by each other - which means that A) I can't see any health bars but the one in front, and I can't select them to apply stimpaks! I've had a crazy number of dwellers die in molerat and deathclaw attacks, who I could have saved except that the game literally prevents me from being able to tell that their health was critically low, and even if I could tell I couldn't select the dwellers to heal them!
The game should NEVER prevent the player from being able to see dweller stats or apply stimpacks when needed.