In my opinion, the disadvantages of Bitcoin would be:
• The unstable value of it, may go pump more than we know, or go lower than we could ever imagine.
But I think there are also some positive results that is happening because of it's volatility. You can see that when the price dumped when it first breached the wall of $11,000 dumped back to $8,000 and then pumped back all the way back to $11,000.
• Security issues, hacker would try and attempt stealing Bitcoins espcially if it's value continues to pump up.
I think this is not a disadvantage of Bitcoin but this is all on the user because at the end of the day, they are still the one taking care of it's bitcoin and bitcoin wallet, thus they are the one responsible for that wallet to be hacked, users must really take care of their wallets.
• Fiat money, it's unlike paper money to you get to see and touch everyday lol
The reason it is digital, and it is decentralized so I think that is not also a disadvantage.
• It's used for some illegal transactions happening around the web or internet
There are some threads here that discussed that and in the end they found out that bitcoin is not that really anonymous and only a little percentage of people use bitcoin in illegal transactions.