In my case, Bitcoins are not accepted widely, I think that would be the biggest disadvantage that I can see. Yes, Government is still in the process of adopting it but like myself, I could not fully feel its greatness that would also let me experience to understand more. This is one reason not to rely completely on Bitcoins.
Another thing is yes I agree that everything that was ever made has its flaws. I think that many for sure are insecure of the fame and wealth that Bitcoin have. What if a specific flaw is seen by someone who is willing to make it as a way in ? Scary right ? That is why it is also a disadvantage that it is digital currency.
It actually doesn't matter that much that bitcoin isn't widely accepted because we have a exchanges. When you're going to buy something, simply use an exchange so your money can be then withdrawn via ATMs. After all, once bitcoin is widely accepted, the prices would still be based in fiat. It's basically the same thing. You're just indirectly spending your bitcoins as of now.
Someone can be "insecure" about the fame and wealth of bitcoin but they can't do anything about it. Bitcoin isn't some entity like the famous people out there. For an instance, you got insecure or jealous of your friend because she is famous and wealthy. Then you can defame her by spreading humors and whatnot and that would directly affect her. In bitcoin, all these FUDs are common and irrelevant. People only base things on facts.
PS: If bitcoin has a flaw that can lead for it being destroyed, altcoin devotees should have found it by now and spread it non-stop.