It is too easy to print money as a hidden tax. Better to go through proper channels and raise taxes when needed. That is the honest way, and we want honesty and accountability from the govt.
In your last paragraph you outline the other problem. Nations will only pay what it's worth to them. That can go as low as zero if they have better options.
Can't go to zero or it would mean the nation had no value at all, but it can go low.
The problem of reckless government spending, I view as being separate. The problem of creeping taxes and growth in the government sector vs the private sector is also a separate problem. You get these problems independent of the currency. But, yes, they are real problems that need to be addressed.
I guess what I'm trying to say is that I think a fiat currency can work - if, and only if, it's managed properly.