. . . is that they're hard to abandon in a hurry if the modern equivalents of the Huns, Goths and Vandals come over the hill, screaming war cries and brandishing weapons. This converted Atlas missile silo is a perfect example.
Florida developer Larry Hall is creating a 15-floor complex with facilities and supplies to support between 36 and 70 people for more than five years while off the grid. Residential condos are available in full-floor and half-floor layouts, with a full floor housing six to 10 people and the half-floor housing three to five. Residents have also been promised access to a zero-edge pool with wall mural, hydroponic food-growing area, exercise facilities, classroom, library, a movie theater, surgery center and an in-house dentist/orthodontist. To monitor what’s happening on the outside, the facility also has a Remotely Piloted Vehicle, equipped with light and thermal-imaging cameras and a military grade security system with “both lethal and non-lethal defensive capabilities.”
There's more at the link.
At $1 million for half a floor, or $2 million for a whole floor to yourself, it's not exactly a cheap option. It looks even worse when you consider that every single floor is underground (the lowest level is about 140 feet deep), and has no egress to the surface except up through the silo's other levels. If someone lower down is injured, and no power is available for elevators, it's going to be hard to get them to the surface to evacuate them for treatment - assuming evacuation is a possibility, of course. (In a major emergency, it may be out of the question.)
Furthermore, in a major emergency, those looking for shelter and supplies will probably know of the existence of this facility, and come knocking at the door, demanding admittance and/or a share of what's stored inside. Refusal will invite attack. If intruders occupy the surface buildings, those in the converted silo will be trapped inside. They may be able to stop the bad guys getting further in, but they won't be able to get out. Furthermore, if the silo's long-range sensors should become inoperative (and anything mechanical will break down sooner or later), they won't even know that the intruders are coming until they're literally on top of them.
In a real disaster situation, where law enforcement assistance isn't available, intruders would have many options. If they have enough supplies for a while, they can sit on top and starve out the defenders. If they need the supplies inside, and are desperate enough to be willing to take casualties to get them, they can fight their way down the interior levels (particularly if they have the means to breach doors and walls, such as dynamite, angle-grinders, etc.). If they take hostages in the first couple of levels, that will help them 'persuade' those in lower levels to give up without a fight. If the silo's supplies weren't essential to them, and there was a handy river or dam nearby (and at a higher level), it might not be a major problem to divert the water into the silo and drown everyone inside. A few sticks of dynamite and/or a backhoe might be all that's needed.
I've written before about why bugging out is not always a good idea or a worthwhile option. This concept suffers from all those concerns, and adds more. The silo may be weatherproof, bombproof (except against a direct hit from a bunker-buster or a very near miss from a nuke), radiation-proof, etc.: but it's not proof against even moderately organized human attackers. This would not be my first choice for a safe and secure (not to mention cost-effective) bug-out location . . .