Echo the above two posts...
There's also a logistical problem, as you need to carry two lots of spares, for the tractor
and pusher engines, and of course the propellers will be different too.
Also, the arrangement of the engines in strut mounted nacelles is more draggy than
wing mounting, but to do that the pusher engines need to be mounted at the trailing edge,
which can mean more structural weight, along with aft movement of the CG, or forward
within the wing, which solves the CG problem, but then you need extension shafts,
meaning more complexity, possibly more weight, or at least no weight saving, and again,
more spares.
In addition, in designs such as the SNCAC NC-140, the engine nacelles are usually used to
house the main undercarriage, which makes them larger than they really need be, again,
more weight, drag, etc.
Cheers,
Robin.