If he came straight out of SE Asia, he'd be seen as an out of his depth interloper, I think - kind of the flip side of when Leese went out to India/Burma. Fairly or unfairly
If Slim had come to the command from Europe or Africa, he'd probably been a great success. Intelligent, confident enough to own up to his own mistakes and learn from them, well able to communicate effectively with a wide variety of soldiers.
From what I understand, even Stilwell thought well of Slim as a commander, and Stilwell didn't think much of anyone. I wonder how he would have dealt with difficult personalities like Patton & Montgomery.