The walking/running animation really makes it look like the controls are loose.
The jumping looks floaty.
Platform edge detection seems weird/off.
Multiple weapon types look randomly designed and remove the pure symmetry of the game.
Lots of weird and apparently barely playtested maps/modes.
Animations and character models don't look like they are pixel perfect matches to collision maps (like, say, Super Meat Boy).
Listening to the interview with them, they had a lot of the same kinds of problems that Tarn Adams had with Dwarf Fortress: they don't have much formal training and are re-inventing a dozen wheels in a naive fashion.
Listening to that interview, I worry they were accidentally successful with Nidhogg 1 and have no idea why it was popular.
They DIDN'T change the few mechanical problems of Nidhogg 1 (the spawning problem), implying that they don't recognize why it's a problem.
Lots of glitches imply a rough engine (based on the release schedule, I don't think that's an unfair criticism: it's not that far from release).
It looks like all those garbage knock-off SNES platformers in the 90s.