The major disadvantages of blockchain technology are
complexity: blockchain is a rather new technology and there are so many technologies like cryptography, networking are involved in blockchain which makes it difficult to adapt.
size and storage: blockchain is a peer-to-peer network and the nodes are scattered all across the globe this makes them huge and hard to maintain. All the nodes in a blockchain store a copy of the database sometimes these databases can be huge and are difficult to store.
scalability: It is difficult to scale blockchains horizontally because different nodes on the network have different computing power.
regulation: Blockchain is a disruptive technology and has the ability to change traditional banking and monetary systems. So many countries and governments have laws that do not allow the use of blockchains in some forms or other.