I think that one of the biggest disadvantage of bitcoin is that, it can be use on llegal transactions like selling or buying smuggle goods or even illegal trading on black market. It can also be use on illegal laundering. The transaction are very secure and private, that is why it is prone the illegal activities.
first of all bitcoin transactions are not as private as you may think so it may even be pretty easy to track them in some cases. and that is why bitcoin is not used for anything illegal on large scales. there is always some usage which you can't do anything about.
secondly i wouldn't call that bitcoin's disadvantage. literary anything in the world can be abused, specially currencies and most of them are already being abused and usage doesn't define the tool.