why Duke nukem is bad?

It's terrible.

Because its poorly made, passed from dev to dev after the first team spent a decade screwing it up, the texture quality is low, the guns are really garbage save for maybe two the humour is entirely just references to things, not jokes just references, just pointing to something that you would recognize and thats it.

there are more reasons im just really tired and thats what comes to mind right away.

I enjoyed DNF quite a lot. It probably helps that I got the game super cheap during a sale quite awhile after release, and went into it expecting it to be awful after all of the bad reviews. Going into it expecting it to be bad, it really shined through as an incredibly underrated game. Sure, it will never reach my favorites list and won't get another playthrough from me for at least a few years, but I had a lot of fun with it.

The game is fine if you take it as it is, a retro throwback with retro mechanics, minus some bad console influences (but even that has been patched to some extent.

Half-Life 2 wasn't bashed for it's platforming and puzzles, I'd say treat DNF equally as well then.
I had fun with it.

Most of the levels were pretty bland, shrinking gimmick was overused, jokes were lame (but that's really part of the series), and Duke himself somehow turned into a total asshole instead of being an egotistical jerk with a heart of gold.
That last part pissed me off the most, and the key point could've been avoided by shuffling one level with it's plot points around a bit.

It also was a technical mess, but I expected it would be so I could look past that.

It's not as bad as it could've been, but it wasn't good either..

I think Duke Nukem will always be cool. I haven't played much, only Duke Nukem 3D. Here's a fun video.

One reason why Forever is worse than 3D