Originally Posted by wilp888
The biggest disadvantage in entering a country without using your U.S. Passport is that in case of an emergency, you may not be able to get assistance from the U.S. Consulate. In some countries, if you enter using a local passport or Identity Card, you are treated as a National of that country and denied access to U.S. Consular personnel should you need them.
Yes, that seems like one of the biggest potential disadvantages. In my case, if I go to Brazil, I don't think I would have a choice, though, it's something to have in mind, just in case.