Most migration problems start with the assumption that everything moves the same way. It doesn't.
- Products, variants and images move well, but options often need remapping. Configurable or bundled products from other platforms may need rebuilding as variants, metafields or bundles.
- Customers: names, addresses and marketing consent move. Passwords don't, so every customer sets a new password or activates their account. Plan the invitation email and the message on the login page.
- Order history usually comes across as archived records for reference and reporting, not as orders you fulfil again.
- Reviews need their own export and import: Shopify has no built-in product reviews, so they move into whichever reviews app you use.
- Gift cards and store credit need their own step. Customers notice missing balances quickly.
- URLs change almost everywhere, because Shopify uses fixed prefixes: /products/, /collections/, /pages/ and /blogs/. Every indexed old URL needs a 301 redirect.
- Blog posts move, but the links inside them still point at old URLs until someone updates them.
- Apps don't move at all. Each plugin or extension needs a Shopify replacement or a decision to drop the feature.
SEO safeguards. Keep a full crawl of the old site. Redirect each URL to its closest match, never the homepage. Carry over page titles, meta descriptions and category text. After launch, check Search Console for new 404s daily for the first fortnight. The guide to keeping SEO through a migration goes into the redirect work in more detail.