Bitcoins biggest vulnerability for me is security. Given that this is used as source of income or even source of payment for sure there are a lot of ambitious people who will hack and get something from our funds. Hackers and amscammers are known here so for me security isthe biggest vulnerability on bitcoin.
We cannot question the security of bitcoin, security is coming from its own owner. even in fiat, security depends on how you take care of your own money, even if storing in banks, there are some cases that you might lose it if a scammer will find a way to get your money by getting information of you account.
I might consider the possible lost of private keys, which you wont be able to recover once you have lost it. There is no private institution that will give you an assistance or an account which you can easily forget your password and reset it to a new one.