As far as I was aware, all (or nearly all) of the monster effects in 2011's The Thing were CGI. There's some videos released by Amalgamated Dynamics (I think that's the name) of the original work they did for the film as well as the original ending with the "Pilot" alien. Really interesting stuff that also looks really good IMO...but I guess that's why I'm not making movies... lol
As for A:C's effects, they all had a CG vibe to me. CG has come a very long ways in terms of giving the creations a "weight" and a feel of being "present" in the picture and iare generally good but, to me, they always nag me as being artificial. Some of the Xeno movements in A:C nagged me and felt like what we saw in Alien 3. I thought the practical Neo suit designs looked creepier than what we saw in the finished film. So when they reject physical creations and opt for CG compositions, my heart aches.