In CAPM, risk is represented through beta, which measures the sensitivity of a security’s returns to movements in the market portfolio. The model then estimates the required return as the risk-free rate plus a market risk premium scaled by beta. The practical limitation is that beta is an estimate based on historical data and can be unstable over time, particularly when a company’s business model, capital structure, or market conditions change. Different estimation windows, data frequency, and choice of market index can produce materially different beta values, leading to materially different required returns. This sensitivity reduces the reliability of CAPM outputs for valuation and required return decisions, especially for less liquid stocks, newer companies, or firms that have undergone structural change. CAPM is also built on simplifying assumptions such as investors holding diversified portfolios, a single-period horizon, frictionless markets, and the idea that only systematic risk should be rewarded. In exams, the cleanest “usefulness” critique tied directly to the inclusion of risk in the formula is that the model’s risk input depends heavily on beta accuracy.