Hamill looks like a completely different person in each movie iv-vi. I don't know how much of this was due to his car accident or just ageing, but I think it makes it that much harder to get a likeness on a figure everyone agrees is right.
Yeah he has a very unusual face that is just hard to pin down and looks different from every angle.
What I do love about this sculpt though is they still managed to retain his youthful appearance in that movie, whereas a lot of custom sculpts and statues in the past have made him look a bit too old and haggard because of the scarring and what not.