Giving it a bit of thought, I would rule it something like this:
Action - Force Morale. Make an Intimidate check DC=8+ the CR of bloodied opponents. If successful, opponents have the Frightened condition. If opponents are already Frightened, they surrender/flee (DM's option).
Or, you might add in the Wisdom DC 10 saving throw after the Force Morale check. If the target saves, it's not Frightened. Frightened targets have disadvantage on the save.
That would largely solve a lot of the problems IMO.