For those looking for more concrete information about Roman history, a classics professor I know has written a really in-depth two part series about immigration in the Roman Empire over at a new online classics journal called Eidolon:

https://eidolon.pub/barbarians-inside-the-gate-part-ii-c22c5...

Granted, he's writing about Republican Rome so it doesn't directly relate to the 5th century, but it deals with similar themes in a thoughtful way.

And I agree that it would be better for everyone if we collectively just ignored the 2015 incarnation of Niall Ferguson, but unfortunately being a tenured professor at Harvard (and now Stanford) gives you quite the bully pulpit! His first book is actually good, but it's been a downhill slide into largely fact-less political assertions and nasty ad hominem attacks since then...