Last weekend I visited the Brazilian part of this WHS, that is São Miguel das Missões. I had previously visited the Argentinian ruins of San Ignacio Miní, and these two sites are probably the best preserved of all inscribed Jesuit Missions of the Guarani ruins (the Paraguayan ones are a separate WHS).
The former São Miguel das Missões church has a remarkable façade and also a very interesting museum of guarani christian art, including a bell used when the mission was active.
There are some other ruins nearby, worth visiting, but only the São Miguel ruins were declared WHS in Brazil.