No significant difference in leg extension required but it can be a little tricky to get it right, depending on the seat shape and BB height. Sometimes people scoot up or down the seat as they pedal, so an initially-correct leg extension changes during the ride. If so, better to set it so the rider settles into the ‘right position’ for most of the ride.
There is also an adaption phase as people ‘use different muscles’ on a recumbent and those muscles need to get used to pedalling in a recumbent position. This takes months for most folk. HK has gone through that this year with her new-to-her recumbent trike. Her overall fitness hasn't changed much but she can scoot that trike along much better than six months ago.
There is a circulation issue that affects some folk more than others. Leg muscles and associated circulatory systems were designed to pump blood back up towards the heart against gravity. Very reclined positions and high BBs mean some folk find it difficult to fully oxygenate their legs, limiting power.
Semi-recumbent tandem, like a Pino or Morpheus? There is no aerodynamic benefit over a normal tandem and they are often heavier. The captain is probably sitting a little more upright than on a standard tandem, which reduces power from the glutes.
