On the car I installed the kit in, taking the door panels off and mounting the motors was the easiest part. Neatly running wires (I am a bit of a perfectionist when it comes to making aftermarket stuff like this look as OEM as possible) along factory wire harness runs, correctly fusing everything, fitting a lot more wires than were meant to into doorjamb wirelooms, deciphering not very well written instructions, etc. It's not hard, just very time consuming to do it all right.
Edit: I still have the instruction manuals for the SPAL kit, if you are really considering doing it I could scan in a few pages to give you an idea of what you're looking at.