I may be one of the ones who has espoused the use of the threadless setups you are talking about.

We have a thread going on our favorite c&v riders, in it i posted a pic of 2 Italian bikes both with quite similar geometry , And using the same handlebar design. (Salsa Bell lap) But one uses standard 26.0 bars and a Technomic stem for comfort, the other uses 31.8 bars and a Thomson MTB stem mated to a threadless adapter- the 26.0 bars with the technomic are 44c and i have 46's mated to the Thomson setup so the wide bars on both do lay down some extra leverage

There is no comparison - i can feel a lot of deflection with the technomic setup when standing, so much so that i feared i was going to tweak the stem if i had to keep standing on a long climb once---- the flip side is the Thomson setup that makes the other bike feel as solid as anything put out by the S Works guys lately

Probably a more fair comparison would not be a Technomic, but maybe a regular 1A stem or a tig welded stem, but i have a box of tig welded stems that are damaged beyond repair also from flex, to include a Cinelli TIG welded track stem that flexed so much the chrome was flaking off in little strips

Rider size, weight and riding style has a lot to do with this though --- im a trackie, and was built like a stereotypical track sprinter of the mid 90's (5'9 210 at the time at <12% bf ) - Leaning hard into the bars and trying to get 12 or 1300 watts or more funneled into the back wheel is hard on equipment

That said, we all used 1" quill stems back then and lived to tell the tale, ----- but a lot of the little short, steep forged NJS Nitto stems are steel, as are the bars, and a lot of Tig welded mtb stems were used on the track then too

My physique has since evolved, thanks to the liberal application of Anheuser Busch products and leisure time, -- so i am not putting out any real power now, but being larger than average causes me to notice differences in stiffness or flexiness ,

That said, i greatly prefer the look of the Technomic equipped bike, and it doesnt get any better than a traditional Cinelli setup, with the deep graceful round drops, - but progress is progress and the new stuff works better. (Edited -- should say --"works better for my needs")