Isn't a lot of this due to indigenous groups converting?
Yes and no.
A member of the Presbyterian Church of Brazil recently started attending my church. He describes the situation as follows.
There are two similar sized groups joining the Protestant churches.
a) Indigenous people. This has a lot to do with historical grievances. They convert almost exclusively to Pentecostalism
b) Working class/poor Hispanics who were mostly unchurched or underchurched Catholics. Their conversions are largely due to strong Protestant evangelizing efforts, and charitable work in the slums. They are mostly Pentecostal with mainline* minority.
Both a & b have issues with prosperity theology in varying degrees.
There is also a smaller group of who are converting for the same reason that many American mainliners left for evangelicalism; lack of rigour in their home church. These folks are mostly middle class and join the mainline churches.
*"Mainline" here refers to liturgy not theology. My acquaintance tells me that liberal Protestantism is almost non-existent in Brazil and I assume this is true for the rest of Latin America.