We all know the advantages of BTC, but what are some of the disadvantages??
Well one that i can think of is that it is still not perfectly anonymous. Personally i'd like to see bitcoin evolve into the future into something like XMR with essentially in built coin mixing. This is probably not necessary but rather an add on.
Another thing is that, fees are often high.
Also, blocks are 10 minutes on average in between so that means you're probably not going to be able to get stuff irl instantaneously without the merchant having to trust you.
I must say that BitPay, an API which often used by many merchants widely around the world accepting zero confirmation, I don't know how they can solve the double spend issue in their internal but it's really a good thing.
Talking about fee, mainly because people want the confirmation to arrive fastly so that they set the fee higher than usual, when mass people do this, the fee will increase significantly, i can say that's the disadvantage of blockchain though.