I think the biggest disadvantages of bitcoin right now is that there's not many merchant who accept bitcoin as a payment method. The other disadvantages is the transaction speed, the confirmation can take a lot of time sometimes.
Transaction speed is the major concern and the ever increasing transaction charges will be a problem in the future apart from that i do not find any issue with the coin and majority of the merchant might not accept bitcoin as a payment mainly because of the high volatile nature of the coin.