I wouldn't say there is as much copying as everyone says, in a way every medley has copied k3 to the degree z 3 has, and those aren't judged at all, though this is my point of view.
Differences
For one, the hidden bosses are not hidden bosses from z 2 like the way k3's hidden bosses were from k2
The secret items function way differently than k3, though they may look similar they have different functions.
A bit of a complicated one, but the bosses stay true to their original counterpart for the most part and don't coward off like in k3 (Except Boss 3)
Boss Rush has actual pattern attacks compared to 3 or so in the entirety of k3's boss rush
These are to name a few
As stuff like the blow game screens are the same tilesets and the rooms directly after k3 in the games order, i see why that flak was given, but let's be honest, when was hurtful pain in k3, or hope the, or hell, about 80% of the screens. A few exceptions Humantic is very similar to Black but we can also say it's very similar to Breaking Out in that respect, feels like every medley needs a maze i guess, as for Catastrophe being from k2 yet using different screens, z 2 also used catastrophe from k2 so it didn't fully make a copy of it, C/O/S/M/O and Entrance are completely original choices and to be honest an entrance boss was bound to happen sometime in a medley.
These are my views, feel free to disagree, Z series is just more fun to me, stinkycheese's word on delfruit is honestly the best way i can put it, hes said it better than i have, so read his review, because i will basically be saying the same thing he has.