When Don Garlits ran his Mono wing the car was built to be run as a mono wing, so maybe the balance and stability was better than if it was just stuck on a car built to run a regular strut design. Don likenend it to the rear stabilizer on an airplane, or the feathers on an arrow, without which neither would fly stright. If you see pictures of Top Fuelers from the 80's, the rear wing was much smaller and lower, it has been an ongoing development for them, and the same would be expected for mono wings to be perfected.