BTC bear market allows Nu to sell a lot of NBT.
The reserve rises over the threshold.
An NSR buybacks is done.
BTC market tides turn.
The reserves get emptied.
How do you keep the peg?
By selling a few hundred USD worth of NSR?
Is it really so hard to understand that you don't make money from buying BTC when they are expensive and selling them when they are cheap?
With a sufficient spread you have at least a chance.
The above scenario is even without NSR buybacks dangerous. Buybacks just make it worse.