Lot of countries do not allow to use crypto currencies and they discourage using them. However as far as I feel, trading with crypto is not that harmful for the economy of the country. As an example, if a person has Eth and someone from another country needs to buy Eth, that buyer needs to pay the seller in terms of dollars. Then that seller needs to convert that Dollars to his native currency which means it creates a the demand for the native currency and increase the value of the currency. May be the government is unable to add a tax on such transactions but it is beneficial for the currency used in seller's country.
What do you think about that?
Including the fact that citizens of one country will be able to transfer money to other states uncontrolled for the state, this is one of the reasons why the states do not want to implement crypto currency. About. this was clearly stated by the Chinese government last summer, indicating that it considers the currency a threat to the economy of its country.
Another problem from the point of view of the states is the high volatility of the decentralized crypto currency and therefore a significant level of risk when investing in it.