Sounds like a great idea if you plan it correctly, especially if you are building the home. Drainage is key, keeping dampness out will be full time depending where you live. I fought a fire in a home that had a basement room that had cinder block walls and steel door. Big fire, lots of water that ended up filling that room up. Hard to plan for but something to think about. You might be able to do pre cast panels and basically make it water tight. Second way out is a great idea. If you are thinking wood frame, there is a sheet rock out there that has an aluminum sheet core, that is really hard the get through without the right tools. You could frame a wood floor over the concrete, with vapor barrier and insulation, giving you a built in insurance policy over the occasional heavy rain, broken pipe etc., dehumidifier, ventilation, heavy door. That stuff is all,out there, especially for a safe room/ shelter.