That's one of the arguments against fiat currencies, yes. There isn't material value backing them up, just the government's word that "we're good for it".
And it is a silly argument. What backs up ANY asset is nothing more than the belief by people that it has value. What makes you think the promise to deliver the material backing up the currency is in any way an iron-clad guarantee? How can you be sure the other party actually has the gold to back up their dollars? Backing a currency with another asset like gold gives some people a (misplaced) degree of comfort but only because they really haven't thought it through. Having one asset (fiat currency) based on the value of another asset (gold) doesn't change what gives either one value (belief). Worse it makes it difficult to adjust policy to deal with economic ups and downs.
There are many important things that backers of the gold standard tend to overlook. The foremost is that the promise to (theoretically) exchange some amount of gold for a dollar is nothing more than a promise that can be broken. There is no way to force an unwilling government to actually hand over gold in exchange for dollars. Worse there is no way to truly be certain the other party actually has the gold they say they do. The "backing" is just theoretical. It's an IOU, no different than your "we're good for it" of a fiat currency AND it introduces a host of logistical, policy and administrative challenges.
Incidently, they go up in flames when the government backing them stops acting in a fiscally responsible way and attempt to monetize away their debt.
A gold standard (or equivalent fixed monetary base) does not prevent a government from doing irresponsible things nor does it prevent bad things from happening. Worse it does prevent governments from doing many necessary and responsible things especially during a financial crisis.