Please correct me if I am wrong, but as far as I read into both whitepapers it seems to me that both NuBits and BitUSD won't be able to keep up the pegging if and only if NuShares or BitShares respectively crash to zero value. I don't see how the BTS market rules are able to counteract this effect.
The idea is that by the time it gets to 0, all outstanding bitassets should have already been bought back and covered - you can't have the rug pulled out from under you except for truly catastrophic collapse where traders literally cannot react in time. With NSR you can have insolvency creep up on you without anyone noticing