tl;dr summary: The Green Inferno is like when a particularly exploitative issue of National Geographic has a baby with a gag-laden issue of Fangoria.
It turns out that building a movie from lazy, stupid stereotypes makes for a lazy, stupid film. College kids are ~ignorant and ~arrogant, activists are ~the real problem, brown people are ~savages, women are ~catty, etc. Oddly , the film feels like it was made by someone with very little life experience who is relying on the echo chamber of narrow minds for ideas. Things it appears Eli Roth does not understand: how undergrads talk, how marijuana works, what an interesting film might be like.
Eli Roth's usual mixture of gore and slapstick humor is especially cack-handed here; at every turn it works against the premise of a horror film. I think we're supposed to feel traumatized when the plane goes down in the rain forest, but that's undercut by one character puking on himself during the descend and another RUNNING INTO A PROPELLER like a Looney Tunes character once they're on the ground. I think we're supposed to feel dread when these dopey undergrads are taken captive to be cannibalized, but that's undercut by weed humor, one character beating off in front of the others so he can "clear his head," and another experiencing some very explosive diarrhea. Anything that could add terror or horror or abjection to this tired pile of cliches is undercut almost immediately.