From the wikipedia page:
"When the yerba is harvested, the branches are dried sometimes with a wood fire, imparting a smoky flavor."
"...researchers in Mississippi found that both cold and hot water extractions of yerba mate contained high levels (8.03 to 53.3 ng/g dry leaves) of carcinogenic polycyclic aromatic hydrocarbons (PAHs)"
Duh. PAHs do not spontaneously arise in heat-treated leaves. Woodfire smoke is also responsible for PAH uptake in grilled foods, although the primary carcinogenic compounds formed in cooked meat arise from a process called carmelization of proteins. Eating vegetables appears to mitigate the carcinogenic effects of grilling, as does marinating (different reasons).
The oncogenic effect reported in (epi- and endothelial) cells of the skin, upper GI tract/head and neck, bladder and lungs appear to be consistent with toxic metals such as arsenic that overwhelm natural reductive capacity or supplant selenium at the active site of glutathione enzymes. These metals are bioaccumulated in many plants.
Arsenic contamination of tea has been reported/studied. It's usually found in cheaper teas, grown at low elevation in contaminated watersheds that are downstream from mining areas or as a result of "natural" groundwater contamination in river valleys catalyzed by surface water nitrates acting on natural sediments or underlying rock (nitrate sources include agriculture, urban pollution).
Yerba mate chemistry has not been studied in depth.
An aside: http://www.noborders.net/mate/what.html
"Mate has a characteristic mature flavor which is somewhat sweet, bitter, withered leaf like, and alfalfa-like, similar to that obtained from tea (Camellia sinensis). Of the 196 volatile chemical compounds found in Yerba Mate, 144 are also found in tea. "
1. Kawakami, M. and Kobayashi, A.; Volatile Constituents of Green Mate and Roasted Mate, J.Agric.Food Chem. 39, 1275 (1991).