A Shopify Plus migration that goes sideways doesn't usually fail during the build. It fails three weeks after launch, when a checkout extension breaks during a flash sale, or when the agency that built the site has moved on to the next client and nobody on the merchant's team can safely touch the theme. That gap — between "the site looks great in the demo" and "the site holds up on Black Friday, integrates cleanly with the ERP, and can be handed off without a six-figure re-platform two years later" — is exactly what separates a Shopify development partner from a general web design shop.