My personal lesson from the fermi paradox and the drake equation in particular is that we humans are terrible at grasping very small probabilities. Intuition completely fails when we have to decide if something has the probability 10^-5 or 10^-50, which makes sense because for almost all practical purposes both are almost zero in most situations, but for edge cases like the drake equation they make all the difference.
Its not just difficult to grasp. Its very difficult to estimate in the first place since have very little data at hand to make any call. For example we may take 1/9 for the probability of life appearing in a given planetary system, but we have no idea if this is representative or not of usual probabilities or if our system is exceptional in any way.
