You essentially have to ask the government for permission to do anything with funds in a 401k/IRA. It's not a normal holding. It's already an impractical pain to make various kinds of investments with such funds; the accounts are pretty much all set up for US traded equities and nothing else. But the real kicker is you don't even have some sort of lifetime guarantee on the restrictions. I could easily see them making it altogether illegal to hold foreign assets, for example. The worst case scenario is some Cyprus style haircut.
It seems to me you don't quite own assets in a 401k. You cede some rights to Uncle Sam.
I feel there's enough political risk to socking money away in such accounts to be wary. In the event of a fiscal crisis they're juicy targets.