The only disadvantage I see in bitcoin at a moment is that the usage of bitcoin is very limited and it cannot be used at most of the places for shopping or paying at restaurants as bitcoin is still new to many countries and it will take long time to reach everywhere.
Indeed, although bitcoin is popular at the moment there are not enough investments, like for regular usage which makes it hard for normal people to find a reason to use instead of fiat, and it being used less for things we really need and more for gambling and other things like in the black market, gives it a bad reputation, but I think in the future this will eventually change as it becomes more and more popular.
The biggest problem can arise because Bitcoin is used for purchases of prohibited items, including on the Internet. If such things continue to go on then the governments of countries can severely restrict the rights of users, as well as their capabilities, in order to strongly control the crypto currency or worse the ban.