My late 50s Holdsworth 531 Plain gauge is comfy like an armchair but bendy as anything.
My 1982 531db Holdsworth is stiffer, still comfortable, but gets very scary and out of shape over 35mph if you need to change direction like a downhill sweeper.
My circa 1985
Evans/Saracen 531 mtb tubed mountain bike will rattle your teeth loose even on big fat tyres.
The 1990s Colombus Nivachrome (reasonably stiff for steel - state of the art for road-racing) Omega, I sold would also rattle your fillings over every piece of grit in the roads.
My 853 Rourke custom 'Audax' is stiff and reasonably comfortable with 853 forks (may replace fork for something softer) solid as a rock at any speed BUT has 'Audax' geometry and 28c tryes where other roadies had 23c. A different beast.
But, my 2004 Giant TCR1 (last of the triple-butted Aluminium) was also pretty damn stiff, but fairly comfortable as well on 23c.
So, there's more to a bicycle performance/comfort than its tubing. But all tubesets and bicycle designs have their pro's and cons.