The most common criticism seems to be that he's racist, which is kind of understandable when you can cherry-pick quotes like (paraphrasing here from memory, but only slightly) "The wars between Negro tribes in Africa are of no more import to World-History than the struggle of two rival ant colonies" - until you consider that he thought exactly the same thing about (say) warring Germanic tribes, i.e. his own ancestors, during the Migration Era. And he certainly admired the achievements of the various 'great cultures' that didn't arise in Europe, which is most of them.
In fact he goes so far in the "biological race doesn't exist" direction as to make some very strange statements, including the prediction that 'soil' (land/landscape) is so vital in the formation of the people that arise from it that the descendants of European settlers in the Americas would eventually come to physically resemble the native people they'd largely displaced, which was self-evidently bollocks a hundred years ago and is still bollocks today. And his dismissal of Darwinian evolution, which I can't remember in detail, isn't a lot better than "if people came from monkeys then y r there still monkeys".
Perhaps the biggest stain on his reputation at present isn't any inherent racism in his writing but the fact that racists tend to like him. Which in itself is odd, and makes me wonder how many of them have really read him closely. He was adamant that the West was on its last legs in 1918 and would be all but over by the present age, which makes him an odd figurehead for people who want to lead this great rebirth (which he insisted had never happened in any previous culture, and was intrinsically impossible) of a white "race" (which he didn't believe in).
But this isn't new, as the Nazis also tried to bend his work to their ends, without much success.