my only concern would be from a safety issue
after my last fiasco and being able to examine wreckage close up I would be concerned that in a rollover or other incident that the possibility of the front engine prop doing something ugly to the pilot by breaking away or in some way rotating around the tube separating the front engine from the pilot
but, hey I guess thats why they call them "experimental"
1. Smaller diameter of prop(s)
2. Cheap and ready to mount power units - all pre-assembled - redrive, prop, exchust etc.
3. No torque
4. Only 1 engine out - in that case
5. Better power out for pre rotor
Wouldn't the props' efficiencies be way down, too? The first one is so close to the pilot that (apart from safety and noise considerations) it sees a lot of "dirty" airflow. And the second one is worse off to start with.