Most stationary bikes require more knee range of motion to operate them which can be uncomfortable for some. And, of course, the seat is much more uncomfortable. It is more functional......and "can" have more compression forces through the knee which is often a good thing.
Recumbents have a much more comfortable seat and usually require less ROM. Patients usually can pedal longer because of the comfort. This may or may not be a good thing. It is less functional, but is still very beneficial.
As a side note, back in college I had a recumbent bike in front of my TV. The seat was comfortable enough that I could sit there for decent periods watching TV while doing at least some activity. Made for a good lazy workouts between my hard workouts.