I won’t say anything the planning, but I’ve learned that migrations are usually hard, in the sense that you have to deal with poorly understood, seldom document

I won’t say anything the planning, but I’ve learned that migrations are usually hard, in the sense that you have to deal with poorly understood, seldom documented and, almost always inconsistent data. And at the same time critical since you usually can’t afford much downtime between the shutting down the old system and start using the new.