There are automagic GraphQL layers that sort-of make sense to me, since at least they remove the biggest pain points. But AFAIK they're all single-database.

Actually stitching together multiple services or DBs with it manually seems like it'd be a hellish experience that'd end in a massive data breech or repeated accidental dataloss + restore-from-backup. Or else valid GraphQL going to such a system would be so restricted that the benefit over just using REST (or whatever) is zero.