Virtually all of these currencies are very vulnerable to what's called a "Sybil attack." Bitcoin is no different [1].
Of course, to economists the concept of a "Sybil attack" is nonsense. Imagine if tomorrow the US Congress voted to give every citizen a billion dollars. Would this be a Sybil attack on the US dollar? Or would it be democracy at work? There's no difference. At the end of the day the majority (of the authority) sets the rules and does literally decide what happens.
It is easy to create a crypto currency that is not vulnerable to sybil attacks; define a central authority. The source of the sybil weekness is not being a crypto-currency, but rather being a peer to peer system.
Re:Ethereum Proof of Stake FAQ
by tlrobinson on Sunday August 20, 2017 @08:14AM
(#15058096)
A central authority isn't really required (depending on your definition of "central authority"), just a connection to at least one semi-trusted node. For example, Blockstream Satellite.
It is not in their power to affect consensus rules such as making money out of thin air or stealing other people's money. In that sense Bitcoin and many other cryptocurrency systems are trustless.
It is tempting to trivialize the creation of value in Bitcoin but there is are many diverse interests with an economic incentive to keep each other in check.