A hardship would be defining the brahmin caste out of the blue. The hereditary warrior nobles become kshatriyas, no problem; the free men become vaishyas, acceptable; the slaves? Shudra, who gives a damn anyway? But for creating a caste above the rulers? Why would anyone in their sound mind do that?
Also, the incompatible mythology will be a big obstacle in bringing Hinduism in. Buddhism avoided it by not caring about the gods, and define buddha as a state of transcendence, instead of supremacy like traditional gods.
And also, social rigidity was found out to be a bad thing by the Warring States, and any state that wanted to keep itself above others switched to meritocracy from heredity (expect the monarch, of course), and the caste system would knock any converters out of the competition.