Attribution often breaks after the landing page. UTMs exist in the browser, but the backend creates the contact without source data. Or the CRM overwrites first-touch fields with the latest campaign. Or signup happens on another domain and the session disappears.
A clean handoff
Capture first-touch and latest-touch fields, store click IDs where legally appropriate, preserve source values, and connect anonymous visitor, signup and customer with a stable identity key.
The best attribution system is not the most complex model. It is the one that survives the real user journey.