The nature of SBD is a derivative, being backed by at least x20 STEEM in order to deal with STEEM price fluctuation. Theoretically, if STEEM price goes down by 95%, SBD becomes broken.
This is not how SBD works since the 10% of market cap hard limit was implemented. In fact STEEM only needs to decline 50% from the 5% level for SBD to start to take a haircut, but the haircut is gradual, and the broader Steem/STEEM/SP economy is never threatened. No matter what happens to the price or ratio, SBD never represents a claim on more than 10% of the total value of STEEM/SP (a relatively small amount which is within what we see in one-day price fluctations on a regular basis).
This is a fundamentally different design with much lower (if any) risks to Steem/STEEM than a hard collateral-based system. In effect SBD is not defined unconditionally as $1, but as the lesser of: a) $1; or b) STEEM market cap divided by 10 divided by SBD supply. Essentially all of the price risk is shifted to SBD. That may or may not be a problem for SBD but it is something we can safely try, and if things don't work out then Steem/STEEM/SP is not harmed at all (and SBD is only harmed incrementally by a fractional haircut).