I've said before that you can't really trust anyone when it comes to language learning. In the last couple of days I've had my nose in a few books that have nice little quotes on the back saying how invaluable to the learner.
There are two problems in general, depending on whether the quote is from a learner or a subject matter expert.
With learners, the problem is quite simply that we often overestimate our own abilities. So is it really effective, or is it just a flawed perception on the user's side?
With experts, the problem is slightly different. On the simplest level, the expert can see when all the information is there, but can't honestly say whether it explains it well enough. Any reviewing expert has the same problem as the author: being an expert, he is blind to the difficulties in certain concepts. A book can make fairly large leaps in logic, but because the expert knows what's in the "gap", he understands what the author means, and doesn't notice the hole in the information.
A particular problem here is the use of jargon. I have one Gaelic grammar book that is widely praised as suitable for learners, yet is riddled with imperfect passive dependent forms, based on a very brief outline of grammatical terminology at the start of the book. I don't have a linguistics degree, but I'm better informed than most Gaelic learners, and I still find the book very heavy going indeed.
But at a deeper level, this is a very dangerous situation. An expert knows the information he's looking for, and a book that lets the expert find the information quickly and unambiguously looks the most "correct" to the expert. But the learner needs a more subtly structured, integrative approach. A great teacher will tie all the concepts together as they appear, and to an expert this looks messy -- the information is spread out throughout the book. So an expert isn't just blind to the gaps in an overly technical resource, in a learner-friendly work he is blinded to the presence of the information by the very things the learner needs.
This is why my favourite resources take a lot of flak from "educated" quarters -- the teaching hides the information it's teaching from casual view, but reveals it to the learner as and when appropriate.