For me, the biggest dksadvantage of bitcoin is; it is not widely accepted. Infact, some countries are banned this kind of technology. If you want the list here
https://en.m.wikipedia.org/wiki/Legality_of_bitcoin_by_country. Second, it has no physical form. You cannot spend it ti any physical stores. Only few stores are accepting bitcoin. You need to convert it to cash in able to buy. Third, bitcoin can be lost. Just like physical.money, it can be lost. For example. You have stored your btc at your pc, but the pc crashes. Your wallet is lost and also your btc.
I think the biggest disadvantages of bitcoin is you can't use it in offline store and it still not widely accepted in other country. In addition to that, as of now bitcoin is still not accepted as real money but you can convert into fiat. Lastly, I hope bitcoin will accept in all country and it will also consider as a real money.