gross, serious, significant
The official figures are a gross underestimate of the true number.

slight

underestimate of

badly, considerably, greatly, grossly, massively, seriously, severely, significantly, substantially, vastly, woefully
He realized that he had seriously underestimated their strength.

consistently, systematically
We consistently underestimate the resources and motivation of our adversaries.

completely, totally
clearly
slightly

tend to

sth should not be underestimated
The importance of these feelings should not be underestimated.

Underestimate is used with these nouns as the object:
ability, complexity, danger, effect, extent, impact, importance, magnitude, power, scale, significance, size, strength, value