Kaufman is brilliant but was not involved in the making of this movie.
No, he wasn't, or at least not ultimately, that I know of. He was the first to write a screenplay adaptation though, and I think it's relevant that such a convoluted mind would take to the book, but when Linklater took over the production, Kaufman's version was not the one used. I should make that more clear. I am adding the phrase, "at an earlier stage " to my reference to Kaufman's text. We can read the Kaufman adaptation of A Scanner Darkly in a PDF file online, which may be of interest to you, or to other fans of Dick and Kaufman. Incidentally, I am interested in learning more about the history of Linklater's production, including "troubled story" of the post-production process referred to in a Wired piece by Robert La Franco.
Unlike most films by the Coen brothers, for instance, you don't get the sense that the filmmaker is having fun at the expense of the characters, or that they are beneath him, so to speak.
Heavens no. The Coens are mean-spirited dorks. They and I parted company quite a while ago, precisely becasue of their condescension toward their characters.
At least two critics (Variety, Seattle Post) have wished that the story's inherent political messages were made more explicit. I think they're clear enough as is.
I wholly agree with you. These complaints are naive. The story needs no "updating."

Are you referring to walkouts at Cannes?
Yes I was referring mainly to the Variety article from Cannes--though I think some other reviewers have claimed to have witnessed poor audience responses stateside. Some reviews have been unappreciative, or, more accurately, simply clueless. They don't get it, or they don't choose to get it. And if you don't get it, you're not going to like it. This is going to be one of the year's best that will not appeal to masses of the mainstream audience. As the name tells you, it's dark.