His writing staff frequently cites peer reviewed studies and pull interviews and personal testimonies from career officials working in a lot of the industries his show covers. Although Trumpist populism has reframed "radical left" to mean anything vaguely left of center-right, I don't think there's ever been a segment he's run where he hasn't at least done his homework and validated some of his beliefs.
Hell, I've always been pleasantly surprised by how forthcoming his show is about how many of his proposed ideas aren't easy solutions to complex problems but can instead be seen as constructive thought exercises that help work towards a future solution. I think many of these political media personalities have a general propensity towards viewing their "solutions" as a one size fits all for very complicated problems. I think it requires oneself to set aside their ego to truly admit that their ideas are just that: ideas. As opposed to some miracle cure bereft of all practicality.
John Oliver's show has thankfully always fallen on the more cautious side of that for me.