I have long thought Russell underrated, for the following reason: his technical work in philosophy strikes people who do technical philosophy as less promising than later people. This illustrates nicely the lack of historical self-awareness in philosophers of an analytic persuasion, since he invented most of those problems he didn’t solve. When kings construct, carters find work.
But the most remarkable comment along these lines I’ve ever heard was Brian Weatherson’s “Principia Mathematica was a great project, but it does seem to have ended in failure.”
As opposed to all the other ones that ended in success?