Overall I think the NCEES test was the best. There were some questions that didn't belong anywhere near the AM section.
I think the NCEES test was still valuable in the sense that the format of the questions were most similar to the actual test, and probably gave me the closest approximation of the pacing I needed on the test. However, I felt that the actual difficulty of the questions, for the most part, was on the lower end compared to the actual exam.
For a general example on a scale of 1-10, for all of the problems that I worked and the actual test, I'd put the NCEES practice exam at a range of 4 to 7 with an average of 5, the actual exam at a 6 to 9 with an average of 7, Six Minute Solutions at around the same as the test, and CERM questions at 6 to 10 with an average of 8.5.
Pretty arbitrary there, but just kinda how I feel; yes there was definitely overlap between the NCEES practice and actual exams, but if the actual exam was like the practice exam, I would have walked out of it confident as hell that I did well. Definitely still recommend everybody uses the official practice test though.
Then again, it's hard to tell. We obviously can't compare them directly, and the actual test has so many more factors playing into it like stress and other people in the room, so they could have been very similar difficulty and I just felt the practice was much easier because I didn't have those extra factors playing into how I performed on each. Because of that though, I would say the Six Minute Solutions are probably the best approximation still, because working those for practice had me feeling like I actually felt during the test overall.