Where does this fail? The generalization from a limited test bench like 250 S&P 500 assets is an obvious mode of failure.
Let's stress this mechanism: the systemic complexity of markets outside the S&P 500, with their changing correlations and exogenous shocks, is not captured.
The weak link is the naive extrapolation of the efficiency of a 64-qubit development system on a subset, to the robustness required for global portfolios, such as managing sudden currency devaluations in Turkey.
Strategies based on such interpretations are vulnerable to market surprises.