How much better for dual over single axle?

I have built many trailers over the last 35+ years. Most were tandem axles and a few triple axles with a few single axle mixed in as well. The last trailer I built for my Avion camper was a single axle trailer with extra ground clearance and a 7,000# Flexiride axle. It is hard to fit tandem axles under a pickup truck bed. I just returned, last month, from an approximately 5,000 mile trip with it. It towed extremely well, even in cross winds. If I had my choice, I prefer a lower center of gravity, with a wide trailer, and tandem axles, because they tow well. This wasn't an option on my last trailer so I put extra tanks underneath it to keep the COG as low as possible and I made sure the trailer wasn't too short. The distance between the coupler and the axles will make a big difference in how they tow. The quality of construction also will affect towing, especially if the axle isn't perfectly perpendicular to the centerline of the trailer. This is even more critical on tandem axles (and triple axles), especially if they are fighting each other as they roll down the road.
I have seen trailer in all three configurations that didn't tow well. Usually it is inferior workmanship or design, but sometimes the axle manufacturer is at fault.