i think (as with any herbals ) tribulus must be standardized & assayed for active % of protodioscin ( not only saponins ).
Even ephedra, milk thistle, st.john's worth, ginseng etc are herbals, yet nobody would question them.
if you buy poorly grown/harvested/treated herbs, retaining no active chemicals, you're buying hay, which tells little about the supplement itself.
That is the case with most tribulus.