Yak is underrated and would probably do well in the KHL. He is missing the defensive play and awareness necessary for a NHL player though, hopefully he finds it.
Omark is another example, currently 9'th in scoring in the entire KHL but obviously wasn't good enough for the Oilers at time where they hade huge problems in general. Why? Answer: His poor defensive play and defensive awareness, his lack of size didn't help either.
I believe it has more to do with some players having difficulties adopting to the hockey played in NA, pure skill isn't a problem for these two guys, that's for sure. Sometimes it may look weird though where we have guys on top ten in scoring in the KHL not good enough for NHL, it's rather common though and it will happen again.
Also these are not #3 or #4 line players and when they get stuck there it gets even worse, downhill.