So after attending several classes instructed by CISSP's they all told me the same thing and it's worth noting this for future candidates.
I've been in IT for 35 years and have a specialty in security and infrastructure management for the last 10 years and wanted the CISSP cert and skill set as I headed towards retirement.
I've studied my a** off for months at a time and yet found it hard to do well on any assessment exam or prep. Feeling disenchanted by the entire thing I lose interest only to have some event or opportunity come my way and I get into it again, heavily I might add.
I've never actually scheduled the exam, most in part for the fear of not passing and the cost. I also probably get worried about it being adaptive, and if I start answering difficult questions, I'm only to expect more difficult questions.
Every instructor I've had told me they know dozens of students who all had similar fears and issues passing the assessments. Most if not all of those students eventually took the exam and surprisingly passed.
In short, don't 2nd guess yourself and definitely don't gauge your knowledge from an assessment exam.
There's knowing everything to be a CISSP and then there's knowing enough to pass the exam and get sponsored to be a CISSP.
I can't comment too much on the assessment exam, I may not able to answer your questions/concern but I can share a bit on my journey with ISC2 exams in general.
There are too many resources in CISSP nows, too many exam prep resources because CISSP is so golden, for example like ISSAP, ISSEP, there is no a single study guide able to cover all the domains.
I will look at the domain of the exam and see if that certfication interested me ( or match what I am doing). For example, I was a security architect, therefore taking the ISSAP after the CISSP is making most sense for me and the domains are well familiar, then actually I don't need to study too much.
I envision I will pass the exam, otherwise I won't prepare for the exam in the first place ( or the exam is not for me, then I won't bother). If you have strong determination and you work hard, you will have it.
Next, I try to look at the study guide or study resources, do my research and plan my study work according, for example I have 3 references material and I would say spend X days for domain A, spend Y days for the domain B, say X+Y days needed, then I will schedule X+Y+some day buffer for the exam (after my research and before my actual study start), this will force me to stick to my plan (otherwise I will just be in domain A over and over again) and I force myself not to pay the reschedule fee ( that force me have good Self-Discipline)
if there is official study guide, I will try to finish all the domains and do all the exercise (for each domain and chapter if they have review questions), trying to understand why I am answering right or wrong. At the end, I will aim getting 90% right before the actual exam. Some exams don't even have the official study guide or CBK like ISSEP, then I will just go to the actual exam without the "assessment exam".
Finally if you know the material well, it's not a problem whether the exam is adaptive or not and adaptive is the last thing you should worry about (and actually you may want the exam to be adaptive, at least I do)
It's pointless to test me 100 questions of the same/similar type but I prefer you ask me 50 harder question to demonstrate if I know the topic. (save me time and energy)
You should be focusing in knowing the domains inside out during the your study instead of worry the adaptive test (or the testing format).
During the adaptive test, it is a good sign for you to see more difficult questions, it's not good for you to see "easy" question. (You should start worrying during the exam when you see more and more easy questions)