United states its seats as “fully flat”, and yes, it is parallel to the cabin floor, but since airplanes fly 3 degrees nose up, they are not parallel to Earth’s floor.
These forward-backward seats each lie at the same angle, but they are not flat, and they are not equal. The seats facing backward are 3 degrees angled towards the floor, and the seats facing forward are 3 degrees angled towards the ceiling.
The downside to backward facing seats are its awkwardness and the fact that you may have a small tendency to slide off the seat. The downside to forward facing seats is that the blood goes to your head (unless you put the seat slightly up so your head is parallel to Earth).
Which would you choose?