What are the pros/cons of low/forward powerplants on helicopters? (Original)

The double-decker Westland Wessex has a pair of side-by-side Gnome turbines low and forward (blue rectangle above).

What are the pros and cons compared to the center-high more typical placement of nowadays?

Of the cons is perhaps the longer shafts I drew above, which are concluded based on pictures that reveal the orientation of the engines and pictures of a partition between (but aft of) the two cockpit seats.



I searched for clear cutaways on Google and Flight's archive. Also checked the Wikipedia page regarding the engine placement.

The Wessex is developed from the Sikorsky H-34 that had a radial engine in the same location.
The radial was put down there to keep the center of gravity low with a heavy piston engine while leaving a decent size space for payload on the center of gravity below the rotor head.  

The compactness and lighter weight of turbines made it possible to mount them up at the rotor head, resulting in a cascade of benefits, and as usually happens, the configuration hits a sweet spot where all the competing compromises are in the best possible balance, so the turbine engine/transmission behind the rotor head becomes almost universal.  

In the case of the Wessex, the turbine engines' location is an artifact of its radial powered ancestor and it likely would never have been designed that way if it had a turbine at the start.