There are absolutely times when it is impossible. If you give me screenshot of goon location you're having trouble I'll create video, faster if you add me.
Goons make different moves, sometimes they'll shake their head or move it in difficult position for arrow to hit with assist, but if you wait a bit until they make perfect pose it'll be very easy to headshot them. You have to get used to its mechanics, after doing so you'll never miss headshot, unless you slack off at that moment and look away from the monitor. No matter whether I aimed higher, lower or to sides, I either missed or hit his shoulder/arm. arrow flew away because I was too far away from target (usually above its head), or the arrow hit his shoulder/arm. There were several situations where I was on the same level as the goon and I could not score headshot. I did some testing yesterday on goons in Nightmare mode. Autoaim assist certainly forces arrows to fly and hit a target but this also means at certain scenarios it prevents you from hitting headshots. I have certainly not mastered use of bow but I used it and I think I grasp the basics to use it effectively. See, this is the point I was making yesterday in a different topic about bows/crossbow. In fact if you master the bow and learn how assist works you'll always do headshots. I did headshots from distances without any issues. And you're wrong about it making headshots impossible no matter how good you are at aiming.