The practical differences are consent, blocking and ownership. Third-party scripts are blocked by a meaningful share of browsers and extensions, so their numbers undercount. They usually require a consent banner, and the data belongs to the vendor to change or discontinue.
First-party tools like Plausible, Fathom or a self-hosted Matomo report less and report it more consistently. For most businesses the real question is not accuracy, it is whether anybody reads the reports either way.
Updated September 2026