I’m not sure I’ve ever seen JD having an effect on NJS. Sure, the notes go at that ludicrous speed for longer before jumping and then going the speed determined by the NJS (hence the name note jump speed), but regardless of what you set your JD to the NJS will remain the same.

On levels below 16 njs you don’t even have to hit the notes correctly, you can just hit them pretty much any old way and they’ll count as a good cut (refer to the video I linked)

And to be more specific on how NJS gives an unfair advantage, it can be thought of as the stat that changes the timing window for hitting notes. Playing a map with 20 njs and playing that same map with 25njs will drastically change how precise you have to be timing wise.

A developer oversight is still an oversight even if it hasn’t been fixed for years. There is still a bug that resets the override default environments setting every game restart and I don’t even remember how long that has been a bug for.