In the tricky data migration cases it's not plausible to know every system and be able to modify them (e.g. to ensure that you write and read from both), many o

In the tricky data migration cases it's not plausible to know every system and be able to modify them (e.g. to ensure that you write and read from both), many of them will be outside of your control, likely in different organizations. That other organization will not run the same operation on two sets of data to verify if they match - they'll just import it in their workflow, and what happens there will have consequences immediately as soon as some item e.g. a person is switched to a new system. And it's plausible that you won't have any direct visibility in any flaws in that migration until that person complains to that organization and perhaps maybe eventually they will identify that your data migration is involved instead of simply screwing that person for months.