Maybe a better way to put some Bitcoin proponents' criticism of the Fed would be that they don't like the monetary policy so far actually produced by politics, and would like to use a kind of money whose monetary policy is produced by something else.
I have never come to a particularly clear or detailed position on monetary policy and so I don't know how good or bad a job of it I think the Fed has been doing from my perspective, nor would I assume that Bitcoin or any altcoin is doing a better job. But it makes some sense to me that someone could say "I wish I could use money that wasn't subject to the Fed, because I so often disagree with the Fed".
I've seen some arguments about this that devolved into elaborate discussions about the institutional status and control of the Fed (as this thread might be in danger of doing...), but I've sometimes seen arguments that went in a different direction, where people suggested that it's important for the Fed to be able to set monetary policy for the money primarily used in the U.S., because monetary policy ought to be subject to political control, because it achieves important political objectives.
I tend to suspect that some form of that is ultimately the issue in play at the bottom of some of these discussions: how subject to politics should or must monetary policy be, and how legitimate or illegitimate is it for people to try to use, on a significant scale, money whose supply and interest rate aren't determined by the central bank of the country they live in?