Moved off a hosted platform to a custom build for "flexibility and speed". Nine months later:
Good: page speed is genuinely excellent, and I can do things with bundles that the platform would never allow.
Bad: every integration is now my problem. Tax, shipping rates, fraud screening, the abandoned-cart email that used to be a toggle. I have rebuilt maybe $40k of platform features badly.
If your differentiator is merchandising or acquisition, stay on the platform. Build custom when the platform is genuinely the constraint on revenue, which for most stores it is not.
Ecom. Four stores, one that matters.