probably one of the dangers of cryptocurrency is when people need it more than fiat. when it happens, it is likely the country's economic system will be disrupted, since most of the state revenues are also derived from taxes on fiat. when people prefer to use cryptocurrency, then fiat will not have a price.
but do no think that there will be any wrong effect on the economy of any county if people accept bitcoin. bit i think it will even improve the economic system of a country. because because bitcoin is providing opportunities to get job or make money from trading alt coin.