I will give an example of what I mean and please tell me what I'm missing because this can't be correct. For the sake of simplicity I will only use USD and EUR. So let's say that the central bank in the US raises interest rates. You buy USD with EUR and leave it there for let's say 1 year. You get your nice interest, and decide you want to buy a house in Europe, so you need euros.
Now what happened with the price in that time? Let's suppose the initial ratio was eur/usd=1. Because of the high interest rates on usd, it's value appreciated and eventually the ratio became say eur/usd=0.5. At that point you decide to sell your dollars for euros. Now, not only did you earn money because of the interest you got on USD, but now the exchange ratio also went your way, so isn't this a double whammy?
To use some numbers: you have 100 euros, you buy 100 usd. Interest is 100%, now you have 200usd. You earned money. You decide to buy euros again. Since eur/usd=0.5, with 200usd you buy 400 euros. You earn money again!
I know this is oversimplified but I would like to know the key point that I'm missing. Is it the uncertainty?