The only downside is a -10 vassal opinion malus (plus the -2 stackable modifier every time you give out a viceroyalty).
I discussed the benefits of viceroyalties last time.
This allows me to keep my vassals weaker. Enacts the Duchy Viceroyalties law, so as well as viceroy kings I can now also appoint viceroy dukes.In 1048, the lords of Bengal Empire approved the institution of the Imperial Administration Law. Pushing into the 12th century, expansion into the south of India was slow. The borders of the Mlechchha Empire reached a natural resting point once the option to subjugate other independent Indo-Aryan rulers disappeared because… I was the only one left.
The century following the turn of the millennium was really all about cleaning up the few remaining maharajas in the north of India, doubling-down on the economy, and making efforts to discourage factions from challenging me. Get the rest of the story: The Legacy of House Mlechchha