What are the pros/cons of low/forward powerplants on helicopters?
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.