SBD is backed by the dollar through STEEM and Steemit, in the same way that USDT is backed by the dollar through Tether. If either platform fails, their pegged asset will be worthless. The big difference is that Tether could shut up shop tomorrow, while with Steemit that cannot happen.

You are right that SBD fluctuates significantly more often than USDT in general, but that is likely due to people trading it having no idea of what they are actually selling. Buying SBD below $1 is free money.