They almost always need some access to the outside, for air or water. Regardless, it’s only so strong. Even if it’s self-sustaining, a few guys with pickaxes could get through whatever wall they have in probably a few months, at most. Assuming everything doesn’t collapse, heavy machinery or explosives would make quick work of them. A bunker is only useful if there’s nothing on the outside actively trying to open it up.
They almost always need some access to the outside, for air or water. Regardless, it’s only so strong. Even if it’s self-sustaining, a few guys with pickaxes could get through whatever wall they have in probably a few months, at most. Assuming everything doesn’t collapse, heavy machinery or explosives would make quick work of them. A bunker is only useful if there’s nothing on the outside actively trying to open it up.