Reputation means everything. No one is going to hire you if you have 3 stars, and someone else has 5 stars. And thumbtack offers basically zero protections against someone maliciously, or even just mistakenly, posting bad reviews.
As long as someone has any form of contact with you, they can post a 1 star review and thumbtack will do nothing about it. If i wanted to be a jerk, i could make some bogus email accounts, contact some pros in my market, and then leave 1 star reviews on them. Even if i all did was ask a simple question, like do you work in the 12345 zipcode, i can come back and leave a 1 star review claiming they are horribly unprofessional. And thumbtack will consider that valid and you will basically have zero recourse since you aren't allowed to discuss it with anyone who has any authority to audit the reviews...assuming anyone actually does.
I've zeroed out my budget for thumbtack. I understand, and support the need for valid reviews. But thumbtack does nothing to protect professionals from malicious attacks. It's not worth spending money on a platform where anyone can come along and destroy your entire reputation in minutes.