Biggest disadvantage of Bitcoin for me is rates are still very volatile. and has no legal force.
But for online payments, bitcoin be one solution. besides perfectmoney, paypal or others.
i do not think that there is any big disadvantage of bitcoin.
There is, but it's based on experience. Think that no system is perfect and you shall understand. The longer you used, there comes a time you can see some disadvantage, like the delay of transaction, it's one.