If all countries around the world have converted into cryptocurrency, it will really be a big problem when it comes to paying taxes because the government does not support and can't control cryptocurrency that they will lose a huge amount of budget in making establishments. But when it comes to making transactions it will be really fast and convenient for every purchasers.
Another problem might be the transaction time and fees like what happened in the past. The blockchain can't take the transactions because of the spam so a lot of people are paying big transaction fees so their transactions can be approved quickly and avoid waiting for days. If all the countries immediately convert into crypto currencies, that would be a problem.