While it can be time consuming, reviews are definitely the best starting point- also I would recommend an outside review site like Trust Pilot for that. Also make sure they are "verified" reviews otherwise it could just be one employee at a company posting fake reviews.
https://www.trustpilot.com/review/www.greenbacktaxservices.com
Also check the website and make sure you are happy with the information provided and don't hesitate to speak to the company and the accountant you are assigned to make sure you are comfortable.
A lot of companies hire interns or outsource the data inputting and only have one accountant review the return at the end of the process. This is not ideal and make sure you are assigned to one person who actually prepares the return and can help you every step along the way.
Lastly, feel free to check the BBB listing of the company for extra assurance.
Here is our BBB listing for example: https://www.bbb.org/us/ny/new-york/profile/tax-return-preparation/greenback-tax-services-0121-123684
I hope this helps!
I read the book, US Taxes for Worldly Americans, and the author has a tax service where they offer a free 20min consult. Worth it just to understand the options you have. They were super nice, and I plan on filing through them.