What a real SEO analyzer checks
Crawlability (robots.txt, status codes, redirect chains), indexability (noindex, canonicals, duplicates), on-page structure (title, single H1, headings, internal links), structured data validity, and Core Web Vitals. Anything scoring keyword density or meta keywords is testing signals that stopped mattering years ago.
Why scores disagree
Scores are weighted opinions. One tool treats a missing meta description as critical; another treats it as a hint. Use a score only to track your own site over time, never to compare against a competitor's score from a different tool.
Checker versus crawler versus monitor
A checker inspects one URL and is useful before publishing. A crawler walks the whole site and finds template-level problems - the highest-value category, because one template fix repairs thousands of pages. A monitor re-runs the crawl on a schedule and alerts on regressions, which is how you catch a bad deploy that adds noindex sitewide.
Turning findings into a fix list
Sort by pages affected times traffic at risk. A broken canonical on one blog post is noise; a canonical bug on every product page is an emergency. Anything that blocks indexing outranks anything cosmetic, always.
What free tools can and cannot do
Free checkers cover single-page analysis, schema validation, SERP preview and sitemap or robots generation perfectly well. What they generally cannot do is crawl at scale, keep history, or track rankings over time - those need either an account or a paid tier because of the ongoing compute involved.