Yes, if the Fed were to signal that it was willing to put ideological purity ahead of financial stability, or the maintenance of the full faith and credit of the US government; in other words, if the Fed were to signal its willingness to let the US government default, then buying US treasuries would become dangerous.

But faced with the choice of defaulting on the debt or shutting down the government, both the US Congress and the Treasury appear to lean on the side of shutting down the government. So even in case the Fed decided to become the Bundesbank US treasuries would remain safe.

I distribute. You re-distribute. He gives your hard-earned money to lazy scroungers. -- JakeS