Does buying at RSI 30 and selling at RSI 70 work?

On assets with strong upward drift — broad index ETFs — the classic RSI(14) 30/70 round trip has been a modestly effective mean-reversion pattern: oversold readings in an uptrending asset often mark short-term washouts. But the strategy spends most of its time in cash waiting for RSI < 30, so it usually lags buy-and-hold badly in sustained bull runs, and in a real downtrend RSI can pin below 30 while the position keeps falling — there's no stop-loss in the classic formulation. Our 59-ETF backtests show both faces: respectable win rates, occasional deep losers, and long idle stretches. It's a useful component, rarely a complete strategy.