"Actually, FNC addressed the immigration issue in their exit polling and said it didn't even show up as a concern."
Sure, but they're also not using the exit polling data, because it's unreliable. So we can't draw a conclusion about immigration from the exit polling either.
I'm sure someone else has already mentioned this, but FR ran it's own poll and immigration didn't come up as even in the top 5 as I recall.
It didn't make the top 5 concerns of voters in the majority of polls I saw anywhere. There was a week or two, when the media was focused on it for lack of anything else to complain about, that it might have made the top 5 list, but I rather doubt it.