The question of rigidity or flexibility is complicated. Some bikes have brutally stiff seat stays. This was the reason I bought the cheaper 2016 Cervelo S3 instead of the S5 at the time. The S3 had the stays from the R5 essentially but the downtube of the S5. Best of both worlds. Stiff downtube and chainstays = good. Stiff seat stays = bad. Top tube = ?

I find it hard to enjoy the bunny rabbits when my butt is being pounded. The best modernish bike that I rode was a Felt AR1, the BB was stiff enough and it was comfortable for a racing bike. Maybe a lighter rider would have had a different impression. When buying a bike, this is kind of an important consideration.

My noodle Vitus in the 80's was so flexible that I could shift gears stomping on the pedals. But many people thought those bikes climbed well, I might have been too heavy (168 lbs on a 60 cm frame). It also shimmied violently at speed. Fun.