Here's another of my road bikes...completely different animal, but no chainstay bridge on this one, either. The way bottom bracket areas on carbon frames are shaped and oversized these days, it's easy to see why they're super stiff when pedaling forces are applied. The chainstays are oversized and extremely stiff, too. If you look at the bottom bracket shell on my Pinarello above, there are some things going on with its shape that tell me it's meant to resist flexing and twisting from pedaling forces in the bb area as well. Does that bb shell help overcome any difference the bridgeless design makes? I don't know, but Pinarello introduced it as the "bridgeless bottom bracket shell" in the late '80's:
Pinarello's "bridgeless bottom bracket shell," introduced in the late '80's:
