Okay, you asked. I think 95+% of O/Us look incredibly clunky. I can't shoot one in anything over 28 gauge. My left eye sees that huge mass out there and my brain thinks its a 4 gauge and cringes at the thought of pulling the trigger. To open one in a blind you need an extra foot of open space.
A good side by side looks like a beautiful work of art, and handles like a dream. With the right set up they work equally well for anything. Anyone who can't sight with one should not even be driving a car.
But, sometime after WWII as the greatest generation started making more money, and the baby boomers were at the impressionable stage, and the gun sages started selling the O/U concept, "single sighting plane", and all that rot, and also began talking down about repeaters, and your average anxious to be well thought of and upwardly mobile Amercan shotgunner jumped on the bandwagon with both feet.
You caught the absolute main point of the whole deal. Salesmanship.
Seriously, as long as we are talking decent quality firearms, I don't think there is 1/10 of 1% as much difference attributable to the guns as there is to the shooter behind it. You can get any of the jobs done, with the exception of long range slug work, with any of the different styles. But, that doesn't sell guns either, and that's what the companies and the writers are out to do to stay in business.