The usual argument against 8 .. c4 is that it's just a slightly worse version of the main lines. It certainly looks that way to me. Nothing dreadful but I can't see any reason to prefer over the near equivalent main lines.
Normally to get this pawn structure white has to play Qg4 -> h5 to provoke Ng6, then Be3 to force c4 and then Qg4 again after BxN and fg.
Compared to this black has lost a tempo here put kept his Q on d8 and white's knight on g1. If anything I suspect that this is slightly advantageous for white too - the normal manouver is Qc7 - > f7 then to e8 after Ng5 which then goes to h3 etc.
This is a wash in tempo terms. However it does perhaps give white a little extra flexibility - iirc there is even one line that Khalifman recommends where white plays Nf3 -> g1 of his own volition just to prevent Rxf3 ideas.
He can also perhaps retreat his bishop on d2 where it's a little more secure and props up the Q side. Not sure if taking that square away from the king is a good move though.
The lines I want to work here are the ones with 8.. Nbc6 and 11.. Nce7. Then the 12.. Nf5 stuff is horribly tactical but might be objectively surviving reasonably.
I've thought of trying a small move order finesse with 12 .. cd 13 cd Qc3+ 14 Ke2 Nf5 (to avoid certain lines) but 15 Rb1!?(!) might render this a bad idea.
The line which is frustrating is 12 .. Bd7 because then after Karjakin's13 a4 black can go 13 .. c4 14 Bxg6 fg etc which looks like it should be an improved version of the main line. The bishop can even reach e8. The thing is that it doesn't seem to quite work like that for some reason. Hard game sometimes