Mouse sensitivity in RoSP is scaled by screen resolution, so if you have the same X and Y sensitivity configured in the game settings, your X-Y sensitivity will not be proportional.

For example, if your resolution is 2560x1440 (a 1.78:1 ratio), horizontal sensitivity will be 1.78 times the vertical sensitivity if both sliders are set to the same value, making aiming nonlinear. This decision is baffling considering the game's environment has so much verticality and shooting while jumping, hovering, and dropping are essential skills.

The way to fix this is to scale X and Y sensitivity based on your screen resolution. In yet another baffling decision, the game's sliders only go from 0.00 to 3.00 which is not enough range to avoid requiring users to change their mouse's DPI settings.

I recommend setting horizontal to 1.00 and multiplying vertical sensitivity by your screen's aspect ratio. In my case, I run at 3840x1600 (a 2.40:1 ratio) so my X = 1.00 and Y=2.40 in game. Then, use your mouse's control panel to adjust the overall sensitivity to hit your preferred effective DPI. It's annoying, but you'll appreciate the end result.