After several months on Thumbtack, I have spent 100's of dollars on quotes to "potential clients" but was only hired once. the odds, at least from my experience, is about 100-1 against you.
The clients are not required to give a reason why your proposal was not accepted, you are just informed the job is "closed" or you never hear anything at all. I tested the system by putting all price ranges with no luck.
I suppose since I got hired once, there are some real people using thumbtack but I wouldn't be surprised if many more are just price shopping with no intention of hiring anyone (thumbtack gets paid anyway) and I also believe thumbtack somehow manipulates professionals in some way... No proof but the odds of you making money on this service are pretty slim.