If ACC speed adjustment let to the trailer losing traction and swerving behind him, then there's something not right with his towing setup, even in a panic stop, I've never had my trailer swerve behind me. That's a serious hazard, ACC speed adjustment braking should not cause any loss of control.
I've seen pickups with too little weight on the rear axle lock up the wheels and get pushed around by the trailer especially when the trailer braking is adjusted too light, but with the advent of ABS, that's not nearly as common.
The problem here isn't the ACC braking (even though many car markers will tell you not to use it while towing), it's trailer setup.
