All textbooks are filled with dogma, based on small studies, retrospective and case series, etc. as the reader, especially people newer to the profession or who don't regularly read, you don't know the quality of evidence to support each assertion. Miller is full of half truths and some chapters are simply written better than others. It is not difficult to find examples of this....the problem is the "truth" that one is comparing it to is also an incomplete story as well.
This is kind of a silly generalization, and the assertion assumes that there might be some better level of evidence out there than what the textbook authors are able to find. The reality is that not every concept in a textbook has been subject to the rigors of a randomized controlled trial; that's just the way it is. I didn't write for the 7th edition, but I did for the 8th, and I can tell you that we do the best we can with the evidence that exists. Sometimes the best evidence IS a case series, but it's not like we're misrepresenting that as "truth," (I hope).
Remember, too, that the evidence, even that from the gold standard RCTs, is subject the biases created by whatever outcome variable was selected and whoever paid for the study. The "truth" that we seek, as you point out, is a moving target. The best the textbook can do is present a summary of the best data available mixed, where appropriate, with the expert opinion of the author (many chapters are written by people who publish in that area), and an extensive reference list so you can go to the primary literature and decide for yourself.
I think one area where all textbooks fail is in the currency, searchability, and fluidity that is found in online resources. Wiki, with all its links and cross-references, is a great format. I'm not sure that a big bundle of printed pages represents the future of education or reference. Imagine incorporating full-color photos, links to videos of various techniques, links to NYSORA videos in the regional chapter, hyperlinks from the reference section to PDFs of the primary articles, etc. That would be a rich text, indeed.