For me, the biggest disadvantage of bitcoin is that it can used anonymously. I know how convenient it is to perform transactions over the internet but I believe that because of that many illegal transactions can be done. Tracing the people behind those illegal transactions would be harder since it had took place anonymously. No personal information have been exchanged in the messages because what they only need is the wallet address to perform such acts.
If it was then the international drugs and arms dealers may have chosen bitcoin but still if you look at some studies you will find that it is the US dollar mostly preferred for the illegal transactions.