It seems there was a batch (nobody seems to know how many) of 27" iMac with screen issues.
Apple wasn't particularly quick to acknowledge it (they never are, but that's because they're are investigating and weighing up options, not because they're unwilling to fix them IME), but they have now and so anyone who has one will get it fixed free, and one can presume that any new ones have had the problem remedied so really its kind of a non-issue for almost everyone.
Apple is saying the problems are fixed, so if you believe them there are no problems. I am on my 3rd iMac after the 1st 2 having the screen flicker issue but this one seems fine and I have had no problems(yet). Checkout: AppleInsider | Apple says 27-inch iMac display issues have been addressed
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