I remember the days of silver certificates and the change to federal reserve notes. Both were completely accepted as legal tender in the USA. [I don't know about elsewhere.] Around that time, they changed from copper pennies, nickel nickels, silver dimes, quarters, half dollars and silver dollars, to composite coins. They were all completely accepted, and they still are, but the coins made of copper, nickel or silver are now worth much more than their face value, so it would be foolish to spend them as money.
I recall visiting the UK in 1966, and their huge copper pennies and halfpennies were worth several times their face value in copper; yet they were the normal currency, and it seems nobody was melting them down. The British were very honest in those days.
The cheapest silver dollar bill over $20 now; most of them are more, depending on their year, mint, and condition.
I am not old enough to remember gold certificate currency.