Some faces just pose challenge to sculptors, with no real understandable reason.
As far as Stallone goes though, I think part of it is lack of products and style of artists. I have several of the Rambo pieces from HCG that Edinho Maga sculpted, and while he's got talent and really captures the character essence and a general likeness to Stallone, his sculpts are a bit stylized, not quite cartoony, but definitely not a realist as some other artists. I'd say the same for his Rocky pieces as well.
I have the Enterbay Rambo 3, and aside from maybe slightly undersized eyes, I think they really nailed Stallone.
Sideshow's Rocky and Rambo busts as well as the Rambo PF all have a similar, stylized look and lack the realism of BW or Enterbay, so even if features are done right, something will still feel off compared to pieces that strive to look like a 3D photo.
The PCS Rambo has a really nice overall portrait, but the few pics we've seen make it look like some stage between the cartoonish style of Sideshow and HCG and the realism of Enterbay, Blitzway and TBLeague, closer to the later, but still not quite lifelike.
I actually find the body posture to be what throws me off with this Rambo 3, not the face. From some angles, the arms seem unrealistic in their pose, from others, not too bad. I think part of it is how the muscles are done where his right shoulder and pec meet, Stallone has that old injury from Rocky 2 that's very distinct and shows up in situations like this statue depicts and they just don't quite have it.