Compare data between 2 different schemas
Although the representation of the data differs between the old and new schema the data itself should be the same.  This means a series of reconciliation queries will provide certainty that the migration was without error.

Say you have an Orders table.  The total number of Orders per year, month or day will be the same in the old and new databases.  Similarly the total value of those Orders for each time period will match.  You can expand this idea for orders-by-product, orders-by-customer etc.

If any data was changed in the migration the corresponding reconciliation queries will not return the same value from old and new DBs.  You will know immediately that the problem was with an order from customer "X" on day "Y" for product "Z" and can focus your debugging on that area.

The same approach can be used for any other table.  Take customers as and example.  You can count customers by country, business area, first letter of name or any other differentiator you choose.