It's always manifestly unfair as it's a massively stupid rule.
The idea behind it is right. It came in when there was a tendency for players to feign injury to waste time, interrupt the flow of the game, throw opponents off their game, etc. and it started to get silly. Players still do that.
The problem for me is that there should be leeway for referees to use the rule effectively, I.e. they should be allowed to leave players they think are exaggerating on the side lines longer, let players who they believe are genuinely injured back on the pitch ASAP, even if that is before the game has restarted. If faking injuries means you are down to 10 men for a couple of minutes, perhaps it would reduce the feigning a bit more.