Instrument differences
Major currency pairs typically move a fraction of a percent per day. Metals, indices and crypto move multiples of that. Stops and targets have to be scaled to each instrument's normal range or they simply measure noise.
Session differences
Volatility concentrates in the London and New York sessions for FX, metals and indices. Trading a range strategy during the overlap, or a breakout strategy in quiet Asian hours, works against the market's own rhythm.
Turning volatility into position size
Set the stop from a volatility measure such as average true range, then derive lot size from the money you are willing to lose. The order matters: volatility first, size second, never the reverse.