Currently it does not make sense whatsoever to own swiss bonds with negative interest rates.
As regard to US bonds, it depends of who the issuer of the bond is, but :
- Treasury bills return a very low yield (the current 10 years “risk-free” rate is 2.87% (see here)
- Risky or very risky companies bonds are not yielding much more : for instance the latest bonds from Tesla have a yield to maturity of 6%. That is quite a big risk for not much returns.
So to sum up, the current gains from US bonds are quite small compared to the fluctuations of foreign exchange rates. For instance, between last november and february, USD/CHF went from 1 to 0.92 (that would have been an unrealized loss of -8%).
So my take is that when dealing with bonds, you want them in your local currency if the rates make sense, which is not currently the case in CH.
As for stocks, the more correct (but highly impractical) way to evaluate the value of a stock is the method of John Burr-Willians which states that the value of a company for an investor is the present value of all future cash-flows. (It is very impractical because forecast of future cash-flows are very hard to make, and the discount rate used matter a lot; it is a little bit like the Hubble telescope : change your parameters from 1mm, and you end up studying a totally different galaxy).
So in theory, if the earnings (and thus cash-flows) from a company are earned all over the world, they should not be too dependant on the quoting currency. It the latter loses value, the earnings increase and vice versa.
(However, I would be curious to see what is really the part of the earnings of the S&P 500 earned in America vs what has been earned in the rest of the world. )