
There are two main general issues as I see it: All the set bosses (including the last ones) are kind of pushovers compared to randbosses (or the player!), and enemy AI/pathing/awareness is still terrible. I'd sincerely hope for these or other solutions implemented probably on Insane/Madness.
Problem 1: Wussy bosses
Due to a wide variety of factors including general power creep, new sweeter artifacts/randarts, infinite scaling hurting enemies more than player, etc, all of the set bosses are incredibly easy. In madness particularly it's sad comparing them to random randbosses.
Solution: Forgot who proposed it, but Starkeep has been messing around with an addon that does it and I think it's an elegant and interesting solution: give the set bosses at least 1 (hopefully more for like The Master/Last bosses) randboss trait! That is, add some random class traits on top of what they already have, not replace. This not only makes them much more challenging, but also makes them more interesting and a unique challenge per game. Also fits in with the theme of Madness of bosses all over the place, and doesn't require messing around with each individual encounter

Problem 2: Pathing (Still!)
If you bother being tactical, there's just an infinity of ways to abuse the horrible AI pathing. Even completely without digging, just roaming around corners, enemies just lose track of you and mill about (even with hunted). It's trivially easy to disengage from all but the most wide open/crowded levels and rest indefinitely without enemies being able to find you. Can post video's of people milling about for thousands of turns without finding me if required.
Solution 1: Hackem_muche found the difficulty and made an addon for this long ago, some of which may or may not have been implemented (didn't keep up with the coding, but there were some ambiguous 1.05 patch notes...). Either way, I think he just reduced/capped their tendancy to completely lose track of you when out of LOS... and I assure you that there's no meaningful improvement in current pathing. I propose radically reducing their losing track of you factor, ideally to not at all. When hunted... you should feel hunted!

Solution 2: Alternatively, if you wanted to only localize real difficulty to madness, you could make it so that when you get hunted they maintain awareness (at least for a lot longer) instead of it lasting only a couple of turns.