Generally, I think the "consensus" among competitive golfers is that if there is NO way to reach and hold the target, its unfair.
So, a putt that will roll 30 feet past or back, no matter how you stroke it is unfair to them. If the cup is set three feet from a 3 foot stair step in the green, but you could "lay up" to the high hide for a tap in, then its probably fair, although even some golfers would say you should be able to make any well struck putt. I think a course full of layup putts wouldn't be very interesting, but they are acceptable in limited doses.
I am surprised no one has mentioned fairways that won't hold a shot, but they fall in the same category and only escape criticism if the course is set up by the R&A, USGA or Masters, with a good example being Olympic, where 17 fw was just not holdable with any tee shot through it's length and caused the most problems. Now, if you could lay up with an iron and hold the fw, and still reach the green, no big whup, but the entire length would be unfair, IMHO.
A reverse slope green with frontal bunkers, rather than a reasonable way to bounce a ball in is unfair, but if fw exists in front, its not.
I agree with cary - maybe unfun is a better word. And it must be taken in context, and it is a grey area. As alluded, a few examples of unplayable shots might be loved on a course. The first one on a course might be fun, but the tenth would be unfair!
And while some love the quirk of an unplayable shot, I have to believe that its not golf to its highest competitive level if the course is purposely or accidentally designed or maintained so that a shot from the fw or green generally towards the hole is not possible to execute. (Hazards not so much, but still generally true here as well) I do disagree with the concept that every shot should be makeable, if perfectly executed, though, including putts.