Browsers increasingly limit tracking. Safari caps cookies set by JavaScript at seven days, so a buyer who comes back after a week looks like a stranger. Some ad blockers also block known tracking script names and endpoints.
BrainyMetrics stores the visitor id in a first-party cookie on your root domain and mirrors it to localStorage. Point a subdomain such as t.yoursite.com at BrainyMetrics with a CNAME and the cookie is set by the server, which survives Safari's limit. The script and every request it makes use a per-workspace neutral path rather than a well-known tracker name, so blocklists that match those names don't apply.