The rule I've written is "once per turn per target." Was that the second option?
My problems with the GWF feat during play were notable mostly by the fighter/barbarian's ability to do huge nova damage against enemies that were built to be tough solo encounters (usually legendary creatures) or a strong creature (often non-legendary) with a bunch of weaker creatures. As I illustrated earlier, he took half the hit points of an adult Green Dragon in a single turn leaving only half the hit points of the creature for the rest of the party. This is another factor that is not being discussed is the limited pool of hit points an enemy has to be taken, thus allowing a fighter or barbarian using a feat like GWM or Sharpshooter to accelerate the depletion of that pool in a fashion that allows them to vastly overshadow the damage contributions of the rest of the group. If a creature had a larger number of hit points, the significance of a single nova round would be lessened causing the damage curve to remain a flatter range over time. That is not how monster's are built.
I do not mind the martials using the feats to eliminate multiple enemies. I've found that numerous weaker enemies are usually a larger threat to a PC party. Thus allowing martials to spike damage to eliminate them rapidly does not damage their ability to threaten the PCs because their aggregate hit point pools tend to vastly exceed the hit point pool of a single powerful creature.
At the moment I've decided to run the feats as is and deal with this issue on the backend as a DM boosting AC and hit points in those fights I want to feel more difficult and dangerous. This is one of the main ways I dealt with the Power Attack mechanic in 3E. I want to see if it works in 5E without making encounters too difficult.
To balance the fighting styles I've added a Dueling Master feat which allows a TWF or single-weapon user without a shield to take advantage of the -5/+10 mechanic.
I have a few players that very much like to play martials. They like the thrill of playing a character that uses a weapon to kill their enemies. They also enjoy obtaining a powerful magic weapon like Excalibur or Stormbringer. I'm trying to balance this in a fashion that eases my DM workload, while still allowing my players to enjoy a feat that makes them feel powerful. As most of know, the damage number makes a player feel like their character is powerful. It's gamer psychology to translate a larger number into the fictional world as a more powerful and satisfying character. I don't mind this power being used in the right circumstances such as hacking down hoards of enemies. I find it to difficult to deal with when fighting a singular enemy trying to build the fiction that this single enemy is truly formidable if the players are able to nova on it eliminating the creature in 6 to 18 seconds. It's like imagining Gandalf and the Fellowship making it to Sauron, then hacking him down in an anticlimactic scene in an extremely short fight where no really gets hurt. Who would have enjoyed such an end to an epic tale? This feats often create that ending if the DM doesn't take measures outside the rules to ensure that doesn't happen.