I think it is not realistic to be deaf in the country for every citizen, how much he owns crypto-currencies. Yes, and still recover the tax from it. To be able to collect taxes, the state should control the system itself, where the crypto currency is actually used - and this is not realistic for a single country. Scales are too huge.
I think it is impossible for the governments to control the system of cryptocurrency for a reason that it is hard to control it. Taxes on cryptocurrency in my opinion is somehow good and somehow bad. It is good because putting a certain tax in cryptocurrency can make it more available and accpeted by people because they will surely think it is now a totally a legit thing. Also, tax is a way to help one's country progress for a reason it serves as a fund. Lastly, based on what I read, it can make the market be stable. As for the bad side, not all the people are willing to pay tax.