From a traders perspective one can consider the business of market-making. In a sense custodians are market-makers in a specific market relating to Nu. Nu as a system ought to turn a profit, but that is not the same as custodians earning large profits without risk (that is not a claim anyway). Custodians and Nu shareholders take risk, and if things work out, they earn a profit as compensation.
One thing to consider is that any Profit-and-loss (PnL) is subject to variance. For example a typical trading operation might be profitable, but have a risk attached to it. Even if market-making is profitable, it will have risk. That is true, independent of how Nu operates. However, in traditional markets it used to be the case that MM’s could acquire licenses which in effect where the license to make money without risk. So a MM on the New York stock exchange had a very stable and profitable income stream. Nowadays these are mostly algo traders (“High frequency”).
Profitability of a trading strategy is not just a question of whether it earns say 10%p.a., it is question exactly what the ROI is, and what the risk is. There is a lot things which can go into the evaluation of a trading strategy.
