but never go down below a preset amount

This part is not true. Regardless of what @cryptogee seems to be claiming here, the fact that SBD can, and does, sometimes go below $1 has been part of the design since 2016.

There are no 100% guarantees. SBD is backed to a value of $1 by steem but only as long as STEEM itself has enough value to support it. When there are major crashes in the price of STEEM, as was the case during most of 2018 (and previously in 2016), then SBD is no longer backed to a value of $1 and can go below $1.

SBD aims to provide a stable value which is a very useful thing (which is why the past several months have seen a half dozen or so stable coins released, with a similar or greater number under development), and it does so reasonably well, as long as Steem itself is healthy. Unfortunately Steem is not healthy. It has lost approximately 95% of its value. At that point, things are going to start to break, and they have (and not only SBD either).