Moving off a system is where data problems surface. Field names don't match. A subset of the records have odd data in them. The export from the old platform isn't what the new one expects.
I handle migrations end to end: getting a full extract out of the source, mapping the old structure onto the new one, resolving what's inconsistent, and loading it into the destination in a form it will accept. I reconcile counts and verify the result rather than declaring it finished.
If you're switching platforms and the data is what has you stuck, get in touch.