I don't know if you are very aware of this but I can tell you from experience:
When you got to rush a production that has a lot of shots using CGI, you sometimes have to ask chineese to do the dirty stuff in a very very very short amount of time.
Many big production do this and I suspect that Prometheus avoided it as much as possible since most CGI were set extension (Matte Painting) and MPC is very well known to be at the top in the industry with set extension (they have some of the best Photoshop artist in the world and usually works only with pictures and a few 3D which gives you a much better feelings of realism since it was picture taken on real location just like most shot in the mountains in New Zealand for LOTR)
Also, sometimes there's nothing you can do in CGI for a shot as for example in LOTR ROTK when Legolas kill the Mumakil.
Several artist tried different animations and in the end my VFX teacher told me that they took the "least ugly" because it was just not possible to give something that could convince people enough. It's cool for sure, but the VFX really do SUCKS in that scene.
Also, it is very well known that when you push it to realism, you sometimes should avoid to go too close because it doesn't look real (i.e. Clue in Tron. It's for me one of the best photo realist character and because it is so real it just fail because they've "passed the perfection" while you can't honestly tell what's really wrong. I mean, the facial animation is perfect, the texture is so accurate and the passes and lighting always felt very very solid).
Also, remember we're living at the beginning of Digital Cinema Era and I think that Digital by trying to be so crystal Sharp looking image kinda "kill" the image by removing the organic look grain had compared to noise that is usually even reduced using even more mathematical algorithm with program that are specific to de-noising.