Throughout my life I've owned and enjoyed all his books and I still occasionally revisit some of them. And I think he's on to something, here, but not entirely. I really do believe in the ETH for some of the cases, just not all. And I know for a fact that demonic powers influence humans that are open to them, so in those other cases, there's a spiritual basis for the encounters.
And then, of course, there are the Deros, who knows why they do what they do.