underestimate
n. countablen. a guess or calculation that is lower than the real amount or value. You use this when someone thinks a task is easier or a cost is lower than it actually is.
n. an estimate that is too low or lower than the actual value. Often used to describe a failure to appreciate the true scale, difficulty, or cost of a situation.
The initial budget was a serious underestimate of the total cost.
Calling the storm a minor inconvenience was a massive underestimate of the damage it eventually caused.
The report suggested that the official figures were a significant underestimate, failing to account for the thousands of workers in the informal economy.
From under- + estimate.
Commonly follows the verbs 'be' or 'prove to be'; often modified by adjectives like 'gross', 'serious', or 'significant'.