To answer your questions,

First, the reason behind banning cryptocurrency in one's country is mostly because the government does not want its citizens to achieve financial freedom. They want their people to rely on them and to the opportunities, they are giving them. They don't want their constituents to patronize a digital currency more compared to their local currency. The government is threatened in every way crypto can take their people's trust and funds. Most of the politicians are greedy. They want the good all for themselves.

Second, using crypto as a currency will be quite challenging because right now, there are very limited establishments that are accepting payments through cryptocurrency. The legality of crypto and the taxation plays a great role in why it is disadvantaged. If you will use crypto as a currency most especially to those countries that are not open about it, you can experience some trouble. In addition, most transactions made with crypto are time-consuming and require a little more transaction fee than usual. You'll have to bear in mind the processing time it will reflect and the fee depending on the network and exchanger used. The convenience isn't much really felt, although they are looking into improving it for sure.

Lastly, some people use bitcoin and crypto, in general, to make their way out to money laundering. This is one of the things the authorities are avoiding to happen as much as possible that is why most centralized exchanges require KYC to verify their clients' profiles and to secure that they are not doing some illegal activities. It's easy to money launder when your transactions aren't really monitored because it is decentralized. Hence, some countries decided to ban bitcoin to avoid such conflicts.