The Temperament Talk group has been going over the bias and whatnot for years, although obviously they love the theory and have much respect for the book. I don't have the book in front of me so if you're really interested you can go there and do a search for "bias" or "Keirsey bias".
From what I can remember, the NF description just seems better than all of the others and makes people, especially Feelers, want to identify as NF even when they aren't. For example, the NFs are the only ones Keirsey describes as caring deeply about their relationships with people (as a primary concern). Yet he presents SP as the opposite of NF, as the tactical-minded users and manipulators of tools including people. So an ISFP teacher who cares deeply about relationships and whose relationships and values are of primary importance may not see themselves in that description, even if the overall pattern fits.
And from my personal memories, the SJ section of the book, while accurate at its most basic levels, is rather horribly dour and sour in tone. I refuse to read it or recommend it to anyone else. Keirsey clearly doesn't like SJs and that's okay, because he got the basics right, but it's hard reading. The tone would not make anyone want to identify as SJ, even if the conclusion seems inescapable. Please Understand Me, the original, was much less biased.
I prefer Linda Berens Temperament book.