No, blockchain technology is not safe by itself, what makes it safe and immutable is the way the consensus is achieved and the scale the nodes of the network are distributed. If a cryptocurrency is run by a single organization, it can easily be changed by that organization, and the history written on a blockchain can easily be altered. Therefore, if a cryptocurrency is backed by authorities, those will have complete power over its characteristics and over transactions occurring on the blockchain of this backed cryptocurrency. A system such as that would have been no different from the fiat system we have today. It would have had all the flaws of government-issued money, including continuously expanding supply, inflation, and the enormous power of central banks over the economy and people. It wouldn't be cryptocurrency where security is achieved through unstoppable code and consensus, it would be corruptocurrency where security is broken by default because it is run by corrupted politicians.