No, I see your men the minute they step on my ground. I don't wait until they're on me, shutting off my air. I'm marking their positions and out, stalking.
Thus, once you are outside the bunker and stalking, you are defeating the purpose of a bunker. Correct?
Say I have 10-20 guys with me. Maybe some women and children. We are armed, but you have no way of guessing our intentions. We may have wandered into your perimeter to forage and or hunt. We may wander out again in a day or so, none the wiser of your location. Do you engage?
You are not going to be able to pull a Die-Hard and take us all out one at a time, and an attack on one would probably result in an immediate all out hunt and search for you. Yes you have a territorial advantage, but during the search we will most likely discover your bunker, air-vent system, solar array, whatever, and will now have a reason for attacking it.
We may of course discover it while hunting or foraging. The fact that it does not appear to be a standard dwelling clues us into the fact that you have prepped and probably have valuable food stores, ammo, power, etc.
I am not saying having an underground bunker is a bad idea. I am saying its a bad idea to not have a standard dwelling above or near it. Hell, the standard dwelling could be 100% decoy and you could fill it with a disabling gas - thus killing or putting to sleep any intruders. Point is, a standard dwelling will make an obvious target and they won't be looking for air vents , steel doors or other entries. Hell they might wander straight past your position because they assume you live in the house. Hell, they might see the house, realize the land isn't vacant and leave to forage or hunt elsewhere and avoid a conflict with you.