Checkout customization — Shopify Functions vs Magento module system?

This is the deepest architectural difference. It bites brands with non-standard checkout flows.

Shopify Plus checkout = Shopify Functions:

• Three function types: cart-and-checkout-validation, discount, delivery / payment customization
• Written in Rust or JavaScript, sandboxed, deployed via Shopify CLI
• Sub-50ms execution budget, runs on Shopify infra
• Powerful for: tiered discounts, conditional shipping rates, fraud blocks, B2B PO entry, custom payment routing
• Bounded: can’t add a custom field beyond what Checkout Editor exposes, can’t add a step, can’t reorder steps, can’t change checkout layout beyond Editor blocks, can’t hold-and-resume across sessions, can’t do server-side data lookups longer than 5ms

Adobe Commerce checkout = full Magento module:

• Override Knockout or Hyvä checkout entirely
• Add steps, add custom fields with validators, route based on cart contents
• Integrate ERP-driven shipping rates (live SAP query)
• Surface customer-credit balance live, hold-and-resume checkout across days
• Multi-warehouse split shipping with per-warehouse stock allocation
• Sample-cart workflows (free sample + paid item in same cart with separate fulfillment)
• No execution time limit — anything PHP can do

The Plus ceiling is real. Brands with subscription + one-time-buy mixed, B2B PO entry, multi-warehouse split-shipping, or sample-cart workflows hit it within 6 months. If checkout is your competitive advantage (or part of your B2B procurement integration), Adobe Commerce. If checkout fits the Shopify-Editor-plus-Functions model, Plus is faster to ship.