Browser-only purchase tracking loses orders to ad blockers and closed tabs, and counts some twice. Server-only tracking knows the amount but not which visitor bought.
BrainyMetrics uses both. Server sources (Stripe, Shopify, WooCommerce, PayPal, funnel builders, the API) are trusted for amounts, status and customer details. Browser sources (tracko.js, your GA4 dataLayer) contribute the visitor and session, which pins the sale to the exact click. The same order id reported twice becomes one order.