Well, bosses are a bit harder for sure, but it seems like the pool of bosses you can get per chapter is much smaller than last time because, at least in the second chapter, there were no bosses from the first one. We know there are around 76 bosses, so by dividing it by 4, you’re left with only ~19 or so bosses per each chapter; while they are harder, you can start to understand their strategies pretty easily.
That is only true for the first time you play a chapter. Bosses are "exclusive" for a chapter until you've finished it. After that you have "unlocked" the bosses to appear in the other chapters as well.