I've never rated a therapist but have left negative reviews for medical providers and highly encourage others to do so. The reason I do it is because no one at the medical practice handles patient complaints adequately, so there are no other means to get a problem resolved or to improve services. For example, a hospital overbilled me 6 months ago, and no one will address it despite my writing 3 times. It's worse for medical complaints. Any problems with services seem to go to a black hole.

On the contrary, If I have a problem with my car repair, I get a call from the Manager to help resolve.

I definitely use Yelp to help guide me on which businesses to use. In aggregate, they are pretty accurate. One easy way to determine medical providers or therapists to avoid is to filter out anyone with many reviews. If most neurologists in the area have about 10 reviews, the providers who have a hundred reviews are using fake reviews to raise their aggregate star rating by drowning out all the one star reviews. Those are easy to a avoid.

Just mentioning that half of Yelp reviews are 5 stars, so it seems people are more likely to report positive experiences than negative ones:

Factsheet | Yelp

More of my own reviews are positive than negative, and you see per the factsheet that is the case for everyone else too.