I'm pretty sure in 'Incoming' you need to aim manually at targets (specially when using the turrets etc). Maybe it auto-aims to a certain degree when in aircraft etc but ground based vehicles/turrets you have to manually aim.
Many people may not be accustomed to using just the keyboard in games in this day and age and alot of people who played this back in the day would preffer to use the mouse as it's more accurate. But as the game stands at the moment the mouse controls are broken.
So until a fix is offered people who enjoy playing with the mouse as well as the keyboard and remember this game the way it used to play (smooth and responsive) before it appeared on GOG with broken mouse controls should stay clear.
It's about all auto-aim, except for ballistic weapons. You get in range, you aim in about the right direction, and it hits. Besides, I have absolutely no trouble at all using the mouse, completely smooth aim. The game crashes everytime I try to switch to the second letter of a highscore entry after a level, though, but as long as you can live with changing only the first letter ever, there's no trouble at all.
That is... in Incoming Forces. The original Incoming doesn't work at all yet for me, gotta tweak the speed down a few pegs on occasion, so the level ist over as soon as it has loaded.