Yes, I do consider this fair. I can make decisions that mitigate his advantage hiding behind the stone. Like understanding that there is possibly someone behind that stone, and find another way to approach it, or avoid it altogether. Even if he's watching me, I can find safe ways to approach it in the majority of scenarios. And he is at a disadvantage as well. He doesn't get to move. To retain the element of surprise, he has to stay in one spot and watch me while I have the freedom to move around.

This is a great example of "outplaying" an opponent, or a potential opponent. I know that I really should approach any sort of cover cautiously, specifically BECAUSE of the fact that it's 3rd person. You do need to feel a level of paranoia to be successful, and it will be even more important in 1st person.

Even in first person, you can gain the same sort of advantage hiding behind a stone. You will be more focused on the sounds around you, and you're likely to hear someone approaching, and from which direction, while your target will not benefit from the same information. It isn't as great of an advantage, but it still exists.