I would think the sheer size of such a country would make it very difficult to govern at first. It makes little sense why La Plata/Rio da Prata with its different economy and concerns should be united with a country centered on Rio de Janeiro. Consider the difficulty of traveling by land from Rio de Janeiro or Bahia to La Plata, and then consider the rivers of the La Plata basin allow for easy access to the entire south of this Brazil. I could see the south of Brazil (Argentina, Paraguay, Uruguay, Rio Grande do Sul, and possibly Santa Catarina and Parana, although this state would be able to easily control the interior parts of Santa Catarina and Parana) revolting for independence. If it had the good timing the OTL Farrapos War did, then a larger base for these alt-Farroupilha rebels might have them win. It would be probably be in the interests of European powers to see Brazil cut down to size so attracting a good deal of foreign support is possible too.