1. Internet every where is not yet done.
2. Awareness with Crypto ( but its not a big thing, as most are aware of how to use currency. )
3. Bitcoin is only 21,000,000 and world population is more. ( This can fix by other altcoins same like USD/RUB/CNY etc )
What other difficulties we can face ?
I think that the major problem for this would be that bitcoin runs on the internet and for some cou tries that do not have neccessarily the function to carry this over to their whole nation would not have the full efficiency of using this cryptocurrency
What government will do is that they will collect tax on every crypto transaction, are the users here ready to pay these taxes? The truth is a majority of bitcoin users adopted it because it is decentralized. Governments will drag it to a centralised pattern I think more than 80% of BTC users will not like it to be a centralised currency.
Secondly bitcoin transaction fee and confirmation time is too ridiculous to be used on the spot.
Honestly speaking implementation of crypto could only be a day dreaming. Crypto physically is not capable to be used as streamline currency. It could only be used as a secondary currency.