how is it a less efficient use of space? less safe?
It depends. It wouldn't be less efficient it they all faced to the rear, it would of course be the same.
But if like a train with some forward facing and some backwards, it may reduce space because there would need to be a bigger gap between facing seats for two sets of legroom.
Seats that were back-to-back, would still need the space to recline and would be naturally spaced a little anyway because the backrest is never quite vertical.
By all facing the same way, the recline or backrest angle of the seat can overlap the legroom space. With some seats facing forward and some backwards, they can't.
As for safety, that was just another suggestion as to maybe why airlines don't tend to install rear facing seats.