What a question: “How accurate are we in assuming that extraterrestrial lifeforms are humanoid?” Not even all life forms on Earth are humanoid, and you ask this question! The insects alone have far more biomass than we humans. Of the millions of life forms here on Earth, only the humans are humanoid. Why would we expect any different on another planet?
On another, perhaps more disappointing note, however, we don’t even know for sure that any other place in the universe has life in the first place. We suspect there’s life out there somewhere, but we haven’t even found an extraterrestrial germ as of date.