It should be fair, in that it shouldn't punish you immensely for choosing one certain class in the game, such as one that specializes in debuffing. A game that Lets you pick a party consisting of three classes out of 6 available classes shouldn't kick you in the nuts/tits just because you chose a support+debuff instead of pure support to go along with your dps and tank. Of course, it's not the same if you purposely choose three classes that do not fill each others out, such as dps, dps and dps.
It should be fair, in that it doesn't have unnecessarily brutal difficulty spikes. Imagine a boss that has a single move that can wipe out every party that isn't overleveled to the point that every other boss can be killed by mashing X. That feels unfair to most players, and also reduces the enjoyment of the other fights in the game because suddenly these do not offer any sort of challenge.
It should be fair in that it doesn't drop instant-kill traps on you that are impossible to predict without knowing that they are there. It's fine to have traps, but they should either be predictable or detectable through game mechanics, or avoidable (at least partially) by reaction/skill.
So yeah, fairness applies to player vs game as well. Games that don't apply it are usually games that people don't bother playing.