I'd think that one advantage of nationalism is the unity it builds among people who share the same nation, who start to think of each other as co-citizens.
One disadvantage of nationalism is the hate can be directed against outsiders.
Some liked the new people, some did not. Some Natives saw this as an opportunity to trade. Some battled with the conquistadors. They were confused because they had not seen someone with the skin color of the conquistadors. The conquistadors also brought along diseases with them which killed many, and made much more ill.
