Criticism related to the Drake equation focuses not on the equation itself, but on the fact that the estimated values for several of its factors are highly conjectural, the combined multiplicative effect being that the uncertainty associated with any derived value is so large that the equation cannot be used to draw firm conclusions.
It is more properly thought of as an approximation than as a serious attempt to determine a precise number.
From the wiki page for the Drake Equation:
https://en.wikipedia.org/wiki/Drake_equation