The most annoying problem of this option is not only that it spends your bonus action on the offhand attack, but that it still does so even if the mainhand attack already killed the opponent in question, completely wasting the bonus action for zip benefit. Thats why by default I always switch it off..
This is what's killing it for me. Forcing you to consume a bonus action for your off hand attack is a questionable design principle. Now you have to play all kinds of silly games to get extra bonus actions, a Frankenstein build that has little to do with the concept of a traditional dual-wield fighter. But burning that anyway if the first hand gets a kill, yeesh. It's bad enough one or two weapons give extra whomp if the target still has full HP, and so they cannot be used as offhand without losing that 90% of the time.
Battlemaster with 2-handers is a much cleaner design. Not that I'm a big fan of all these extra pseudo-currencies.
"Hey, I just had a great idea. Dual wielder, the offhand burns the bonus action, to get extra DPS! I coded it, done!"
"Sounds like a good plan, oh, I hear the ice cream truck bell. Let's go. I hope you remember to think this through later!"
