I averaged 99% across four A-Levels (double maths, physics, chemistry) and used the CGP books extensively in chemistry and physics.
They don't, as you say, contain the full detail necessary to gain every mark. You cannot use them solely in order to attain the very highest marks. You use them in conjunction with everything else.
Textbooks take too long to read. Sure, I did read the official textbooks several times to make sure I covered every point in the syllabus. But for the purposes of rapid pace repetition and testing, your textbooks/full notes are too long winded.
Similarly, past papers are essential. As many as you can, again and again and again until you live and breathe the mark schemes and know the content inside out.
The CGP textbooks were for quick revision. Going through the whole book as quickly as possible (skipping over the jokes), start to finish, repeatedly, as often as possible, to make sure I don't ever forget a topic. I read the bullet point and think of the detail. If I can't remember the detail I look it up.
CGP are great. Not as your only revision material, but as one possible alternative/extra to writing moderately condensed revision notes. For me I had: Textbook > Full Revision Notes > CGP Condensed Revision Notes > Extremely Condensed Revision Notes, + Past Papers/Mark Schemes/Examiners Reports/etc.
I found them helpful.