Why there's no crosshair in the game? It's unplayable for keyboard + mouse with no crosshair.

This. The worst part is that it appears that a crosshair is implemented, in the sense that the game keeps track of the mouse's position on the screen, it just doesn't show it. Since the change in shooting angle when you move the mouse depends on how close to the character the mouse is when you move it, this does indeed make the game quite unplayable with mouse controls, since you cannot judge how much a given movement will change the angle.

I agree, sadly totally unplayable with keyboard + mouse