Vulners Scanner is developed by Vulners Team, the founders and maintainers of one of the world largest security databases.
It implements technology of passive vulnerability scanning based on software version fingerprint.
Is it legal?
Absolutely.
The application does not perform any malicious requests, fuzzing or any other interactions that differ from the common browser behavior.
Is it accurate?
Vulners Scanner is using passive method of vulnerability detection, that's why there is a risk of false positives.
Its report cannot be used as evidence of the vulnerability present or as a compliance check.
For the best result found vulnerabilities should be validated with user interaction using advanced software like PortSwigger Burp Suite with Vulners plugin.