The stuff I've done in Max (just one game prototype) ran on my Mac mini without any changes.
In my experience, it's GUI stuff that needs the most adjustment when going PC->Mac. GuiFont sizes are different and gadgets behave differently (e.g. buttons don't Y-scale and radio buttons aren't actually grouped when assigned to a panel so you have to manually unset others when setting one). Nothing major, but lots of little things ("fiddling" is a good term for it).