Different scanners vary in coverage and false-positive rates; axe-core (which powers many browser extensions and full-site crawling tools) is widely regarded as one of the more accurate and is used inside several major browsers’ own developer tools. Full-site crawling scanners can check thousands of pages automatically, which is useful for large sites, but every scanner flags only rule-detectable issues and can both miss real problems and occasionally flag non-issues that need a human to verify.
A scanner report is a useful starting point for prioritizing manual testing, not a finished accessibility audit on its own.