I think the custodians work on the spread in the orderbook. So even when they had to sell their nubits and got btc for it on the way the btc price went down, they will get btc for cheap when the price goes up again. Of course... its not an endless system, when the bitcoin price is crashing too hard then its a problem. So far the nubits have a stable price of $1 and each custodian providing liquidity to btc gets around 13% each month for his services. That should be the reward for providing liquidity. I think this reward backs the bitcoin price falling till now. So i think custodians did not really lose value at the end. Maybe in short timeframes when the bitcoin price crashed hard but not in bigger timeframes.
I think the point that there are enough custodians should show that it is rewarding for them. They wouldnt do it when they would check their wealth at the end of the month and they constantly would lose by providing liquidity.
I think the nubits price is not only established by the backing but by trust too. Seeing that bitcoins are completely a trust based currency, having nubits custodians and trusting its value, so no rush happens, is a good thing. Lost trust or hedging against bitcoins are the risks. But so far bitcoin price couldnt fall hard enough to make custodians lose their money. That the rewards paid to custodians is worth the same like all nbt is only trustbased, like its with all pos or pow coins. So the custodians earn more wealth over time.
Sounds like a not so bad system. For sure, its no fun knowing that you dont know how many custodians are there and how much btc back nbt at the moment. That would be interesting to know. At least they still live and it doesnt look like bitcoin price will fall anymore very much.