I think what people are missing is that a manual isn't a stand-alone document. It's written and designed to supplement a specific course in a specific way. That course delivers other knowledge via the instructor.
As an instructor, my attitude to course manuals is that they need deliver only a baseline of knowledge prior to my instruction on the course. I expand significantly on that baseline. That's what the instructor is for.... personal expertise and delivering knowledge derived from that.
What I consider a 'bad' manual is one which undermines my subsequent teaching. That can be through incorrect or less than optimal information, bad illustrations (poor role-modelling) or it's just so boring and irrelevant that it switches the student off.
Most of the 'bad' manuals that I see are devoted to specialty/specialist diving activities. They suffer from a lack of specialist expertise from the author/s. Some become 'bad' because they are not updated - practices, protocols and approaches move forwards, but the manual doesn't.
The best manuals support and encourage further teaching by the instructor. They empower the instructor to deliver content relevant to their individual students. When looking at manuals in relation to training courses, sometimes less is more.