I've run and played a lot of Living campaigns, where you get a mix of people and parties of varying degrees of system mastery. Some with internet-derived cookie cutter optimal builds, some with characters they built on the drive to the con, some with characters designed to cause heart attacks on char-op boards. So even for one-shots, I'll still try and work out what sort of experience each player thinks is fair / fun. For one person "fair" means a tough fight where I try all my GM tricks to win combats, but for another it means not KO'ing their bear companion even when it makes sense to do so because that young person is completely invested in the creature.
Fairness is not assessable in a vacuum. A fair combat against a group with high levels of system mastery can and should look very different to one against the people playing an all-bard rock nostalgia band (yes, I ran the same adventure for these two groups in the same con). To emphasize
@Emerikol's point: It is really important early on to establish the ground rules of what is considered fair. I've run a 13th Age campaign where, up front, death was ruled out as a possibility. I've been in a 4E where we carefully defined the level of system mastery we were OK with: Optimal builds without radiant or frost shenanigans. I've taken a player aside and asked them to de-optimize their build as it was getting hard for me to make a fair fight that involved everyone in the same combat.
I always want fairness, but only when the players are roughly equal in ability do I want equality.