Isn't buying currency with high interest rates a double whammy?
Yes, it’s the uncertainty. If you tried hedging the currency, the hedge would be generally such that there would be a loss that offsets the interest advantage. (This is known as covered interest rate parity. In oractice, there is a spread that causes behaviour to deviate from that -the currency basis. So you can lock in a small spread, but it is typically on the order of 10 basis points or less. During a financial crisis, the basis can get relatively large.)

If you do not hedge, the volatility in the currency rates over one year is normally greater than the interest rate differential. Buying a high interest rate currency is known as a carry trade. It is profitable until it isn’t. For an example, look at the Japanese yen in the early to mid-2000s. In that period, Japanese interest rates were the lowest in the developed world. It was thus a popular currency to short. It would then have periodic bursts where its value would shoot up, forcing shorts to cover.