The issue is not FOREX vs other asset classes, it is FOREX Futures vs FOREX cash.I see no advantage of the futures contract over the cash.
As one who trades both the reasons I sometimes use Fx is for the much larger liquidity, ability to put on just about size I want, and having access to many more currencies. Using both futures & Fx gives more flexibility as to how I can trade currencies. Of course you should use a regulated Fx broker with a clean regulatory record.
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For the retail trader, except for possibly bitcoin, all instruments have plenty of liquidity with relatively tight spreads. But within an asset class, often the futures is cheaper to trade, or the cash is.