Underestimate

[Britney]

To set to/ low a value on; to estimate below the
truth.

[Greta]

1. Rate something lower than [its real]/actual value , under appreciate.
   2.[Estimate]/ rate someone lower than his/her actual potential, [qualifications].

[Donald Trump]

Estimate (something) to be smaller or less important than it really is.

[Magaret]

to fail to guess or understand the real cost, size, difficulty, etc. of something: