Kesselring was a LW comander, placed in command of a ground army in Italy. Did okay.
Mountbatten was an Admiral, placed as the overall commander of SEAC, a combined operations post, and one in which he excelled. He had such lumiaries as Stilwell and Chiang Kai Shek to deal with. No easy feat.
Mac ran a fiefdom in the SWPAC, in command of air, naval and ground assets. He was a US Army general, in charge of mostly foreign troops and assets. Whatever his criticisms, he was effective.
Area commanders were not a new concept. What was astounding and different, from the allied POV, was the Joiont Chiefs, which took Joint Operations to a whole new level. SHAEF was a product of that combined operations approach. it cut across national and service divisions very effectively. Not perfect....as the legendary rivalry between monty and Patton clearly shows, but it was a structure that worked, and enabled the best to be extracted out of people like Mallory