Thank you reader Matt, from Burlington, VT for sending me THIS LINK from the LA Times about yerba mate.
A 2003 review of all existing studies on yerba mate, published in the journal Head and Neck, confirmed that people who regularly drink large amounts of the tea -- as much as a liter or more each day -- had significantly increased risk of cancers of the esophagus, lungs, mouth, pharynx and larynx.Yikes. Of course, other environmental and lifestyle factors of the participants in these studies is unknown. (Work past the sensational title of the article, and the researchers quoted say as much.) It could just be a conspiracy begun by big pharma to trash the healing powers of plants, but then I always pick conspiracy if it's an option.