The mouse input feels really odd and uneven in the game. Definitely feels like there's some sort of mouse acceleration or mouse smoothing going on. Furthermore, the y-axis and x-axis sensitivities both feel inconsistent with each other.

There's also some oddity occurring when looking straight up or down while spinning as in the entire screen and mouse locks up.

It's some kind of a feature of the engine, if you look straight up or down, it completely disallows you from turning.

Dunno about the sensitivity.

this can be configured, check the NewDark docs and cam_ext.cfg.

For me as usual, Vsync was causing some slight mouse lag, changing vsync_mode to 0 in cam_ext cfg fixed it.

I believe the different sensitivity on x and y axis is due to widescreen resolutions (won't happen with a 4:3 aspect ratio) looking at the NewDark release notes it was fixed there but doesn't seem to have been here. I've looked at cam_ext but can't see anything there that might fix it.

Hopefully this will be fixed or a way to apply the unofficial 2.4 patch will be worked out