Well a crypto currency can only fall 100%, so it's not unlimited. Moreover, some stocks do actually have a P/B ratio below one, for example Deutsche Banks PB is

Well a crypto currency can only fall 100%, so it's not unlimited. Moreover, some stocks do actually have a P/B ratio below one, for example Deutsche Banks PB is currently around 0.3. You could instead look at downside deviation instead of volatility, but in my experience standard volatility and upside/downside deviation look very similar for most securities. This is somewhat paradoxical for me personally, but it is what it is.