underestimate

To underestimate is to think something smaller or weaker than it is. Underestimate the cost, never underestimate, badly underestimated.

It is the standard word for a mistake of judgement about size, time or difficulty, and underestimating how long a job will take is so reliable that it has a name in project management: the planning fallacy. Never underestimate is a fixed warning. Note the double negative trap — cannot be underestimated is usually meant to mean cannot be overestimated, and says the opposite of what the writer intends.