The addition of national objectives helps the Allies in Allies and Axis Pacific, in an otherwise Axis-favoring game. The addition of national objectives helps the Axis in Allies and Axis Europe by reducing the Allied advantage. If you take these two effects together in Allies and Axis Global, they more or less balance out.
The national objectives in Allies and Axis Pacific encourage the Americans and Australians to cooperate in the South Pacific, which they should be doing, and what they did in the real war. In lieu of these objectives, give the Americans an extra 5 IPCs for San Francisco, and the Aussies 5 additional IPCs for Sydney, so they have 15 instead of 10 to start.
The national objectives encourage the Germans and Italians to play more in the Mediterranean. Instead, just give Germany 5 more IPCs for Berlin, and Italy 5 more IPCs for Rome. Again, this gives Italy 15 starting IPCs instead of 10.
If you add 10 Axis IPCs in Europe and balance that with 10 Allied IPCs in the Pacific while eliminating the national objectives, you've simplified the global game. You have better balanced the two local games as well, relative to no national objectives and no additional IPCs.