The AS400 gets a bad rap. It can be hard to get at its data. But generally, that's not its fault. For decades, the AS400 has come with its own database that works just like any other SQL database. If your programming staff updated their skills sometime over the past 25 years and use the database properly, you should have no more difficulty getting your data from the AS400 as from any Access or SQL server.
But, if you have old code, old programming methods or both, AS400 can be a real challenge. Depending on the age of your system, you may have a lot or a little data stored in "internally described files" as opposed to database tables. And if you have limited AS400 knowledge, this is a problem.