Making a concrete underground shelter is more than concrete walls...
You have to make them waterproof, they need to be strong enough not only to keep out the dirt, but also if it rains, and if you happen at all for it to be in a blast zone of any magnitude, then you have to deal with overpressure...
Of course you cant have a solid wall or celing because you need ventilation, and dont forget cracks from settling.
I think a pre-made (give or take) steel shelter buried is a better bet. Steel will flex in an earthquake and wont crack, if its a rounded shape it will take heavy loads better. Their easier to waterproof and most are galvanized anyway. Drilling holes in the side for ventilation and what have you is a snap, and so is sealing it after the fact.
Eh, to each their own, I would just stress researching the downsides of anything you plan on doing. An unlimited budget, yeah I would probably go with a concrete shelter (or Radius Engineering), but since I DONT have an unlimited budget, I want to get the most bang for my buck but also want to make it safe.
I think of all things a person could get, a shelter would be one of the last things you want to skimp on.