This holds as a general principle, but it's also a principle based on research 60-40 years ago.

For instance in your example, If you bought S&P500 Jan last year in USD and then convert it to EUR in January this year, you would've lost money just because the dollar fell so much relative to the Euro.

You'd still be up on a longer term scale and even if you cashed out now you'd be up (though not 17%) because the dollar bounced back somewhat.

I'd say that this is a good principle but not a certainty and still needs to be considered in terms of a gamble.