Most migration problems are known problems. They are simply checked too late. This list is what we run before every cutover.
Data: every product has a target collection, every variant maps to an option, customer accounts are invited instead of silently recreated, order history is complete, stock matches the warehouse.
SEO: a redirect for every indexed URL, structured data rebuilt, canonical tags self referencing, sitemap submitted, top pages baselined so you can prove what happened.
Operations: payments live and tested, tax rules confirmed, shipping rules matched to your carriers, notification emails checked in all languages, and one person named as the owner of go-live day.
None of this is exciting. That is exactly the point. Excitement on launch day means something was skipped.