For instance how would you calculate fi (the fraction of intelligent life forms), how do you define intelligent? Is intelligent like you and I, is it like a bird or a rat, could it be a plant? There is no way to truly determine what counts and doesn't count as intelligent life forms. There is too much gray area!
Another element to the equation is fc, (the fraction of intelligent species that develop interstellar communication). How would we know which species developed interstellar communication? It would take too long to get out to the life form and then get back to Earth. Once we get out to the life forms how do we know that these certain life forms created the interstellar communication and not another that gave it to them? Researchers on Earth would die before the information could be retrieved and transmitted back. Tying in with this would be element L, (average lifetime of such technologically advanced civilization). Like the fc element you can't calculate how long they live because we will be long gone by the time we get the info back. It would take to many samples to get an educated sample and set year of life spans. Who's to say the rate at which the aliens age? We don't know these things!
For these reasons and flaws the Drake Equation is useless science!