That add-on uses the paid service at rucaptcha.com, which is a Russia-based service. The price appears to be quite cheap, whether for regular Captchas or the more complex modern ones.
There's really no way to know how well these services do, or do not, work without trying them. As far as the NVDA Add-On, it appears to have been around for a while and is one of the "home grown" and not officially vetted add-ons.
Brian - Windows 10, 64-Bit, Version 21H1, Build 19043
Nothing in all the world is more dangerous than sincere ignorance and conscientious stupidity.
~Martin Luther King, Jr.