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.