Lee, I rode my first set of Supremes a lot with four panniers plus extra stuff on long extended trips, but was very, very aware of the thin sidewall and also appropriate pressure for the bike+rider weight.
re squirm-- I would say that any supple tire is always going to exhibit more squirm if under inflated for the bike+rider weight. I found it pretty easy with a small hand gauge to dial in my pressures and I really have to say that with Supremes, my touring bike handled better in corners than any touring bike + tire combo that I have ever ridden over 30+ years of touring.
A combination of tire width, proper pressures and the rolling+ ride characteristics of the tires were all factors in my bike handling so well on curvy downhills, which I love taking at a good comfortable clip.
I'd say that it comes down to the chosing "horses for courses", and any thin sidewalled, supple tire may not be the best choice for sharp rock gravel roads, or for riders who habitually rub the rear tire against objects without realizing they do it.
but we have tons of tire options out there, and decide what trade-offs we want to make for the given use.
My favorite tire combo was a Supreme in the rear and Panaracer T-Serv in the front. 20”, 26” and 700c. With the 20” Bike Friday 1.6” Supreme in the rear and a 2.0 Big Apple in the front.