Detractor — Cambridge
noun Someone who criticizes something or someone, often unfairly
"His detractors claim that his fierce temper makes him unsuitable for leadership."
It usually requires an object, as in "detractor of somebody/something", and it usually does not work as a standalone noun. Max Williams explains in the comments:
We would say "Well, this plan certainly has its detractors, and Steve is one of them." But we wouldn't normally say "Steve is a detractor" - it begs the question "Of what?".
Use the noun detractor for someone who is always critical. You might describe your brother as a detractor of the government if he complains incessantly about taxes, voting, the President, and all the members of Congress. If a person takes a dislike to you in particular, he is your own personal detractor.
Underestimator should work fine. — Wiktionary
noun 1. One who underestimates.