Steel has a very long fatigue cycle, as long as the stresses are kept below the failure point. That means it can be bent (deflected) for years in a bicycle application, with no apparent harm. Aluminum is different, though. ANY bending will work-harden it, and eventually it will become so brittle from the work-hardening that it will crack from a minor shock. So, you can say that by design, aluminum frames are not allowed to bend; which is another way of saying they're engineered to be harsh-riding.

Recumbent applications are a bit different from uprights in that the seat is mounted on a horizontal tube between the two wheels, not at the top of a post that's tied to the rear axle. Between that design fact and the seat padding that most recumbents have, I think the average rider would be hard-pressed to notice a difference in feel between the two.