Not bad at all. Chapter 1 is quite nice and 5 could be in S1.
3 and 4 are quite meaty but the bulk of it is just hypothesis testing.
2 will make you want to kill yourself.
Nah, in all seriousness, I thought S2 would get a lot trickier at times than S3 simply because the wording on S2 was a lot more annoying to comprehend.