Our experience in AoPS Algebra I was that the challenge questions were pretty easy (with no more than 1-2 that required some thought) if - and only if - all the textbook work had been completed first. When we fell behind in the textbook work, e.g. doing the basics but not following through with the full set of harder questions at the end of the chapter, then DS was unprepared for the homework assignments.
I will echo that.
The classes are structured in the following way:-
1. The week before the class on a topic start the required bookwork for the class and do the problems and the challenge problems in the book.
2. Do the Alcumus exercises
3. Do the online class session which will recap the bookwork and address any questions.
4. Do the online challenge problems for the week.
If steps 1 and 2 are done before the class then the online challenge problems are not so bad (with maybe 1 or 2 that may need to be worked on on and off for some days) some weeks.