Disadvantages of Blockchain
Attack 51%.
The Proof of Work consensus algorithm has been used for years to protect the Bitcoin blockchain and has proven successful. Nevertheless, various attacks have been launched against blockchain networks, the most discussed type of which has been dubbed the “51% Attack. It occurs when an attacker entity or organization manages to take control of more than 50% of the total hashing power, allowing transactions to be eliminated or reordered.
While a successful 51% attack is theoretically possible, no such precedent has yet been established in the Bitcoin blockchain. As the network evolves, so does its security, making it unlikely that miners would choose to invest money and resources in an attack on Bitcoin because they could benefit more from a fair trade. Moreover, a successful 51% attack can only change recent transactions and only for a short period of time, since blocks are linked using cryptographic proofs (changing old blocks would require unthinkable amounts of computing power). In the end, Bitcoin’s blockchain is very resilient and adapts quickly in response to any attacks.
Data changes
Another disadvantage of blockchain is that once data is added, it is very difficult to change it. Although stability is considered an advantage of blockchain, this is not always a good thing. Changing data or blockchain code usually requires a lot of effort and often a hard fork – creating a new chane and terminating the old one.
Private keys.
Blockchain uses public-key cryptography (asymmetric cryptography), giving users ownership of their cryptocurrency (or any other blockchain data). Each blockchain address has a corresponding private key. Users are free to share the address, but the private key must be kept secret. Traders use it to access funds, thus acting as their own bank. Losing a private key effectively means losing money.
Inefficiency
Blockchains, especially those using the Proof of Work algorithm, are extremely inefficient. You have to be extremely competitive to engage in mining: for one successful miner, there are many others who work for nothing. Miners are constantly increasing their processing power to have a better chance of finding a valid block hash. Every year Bitcoin requires more and more resources, and now more energy is spent on mining than Denmark, Ireland, and Nigeria consume.
Storing
Blockchain registries can grow dramatically over time. Currently, the Bitcoin blockchain requires about 200 GB of disk space. The increase in blockchain size obviously outpaces the increase in hard drive capacity, and the network risks losing nodes if the registry becomes too large for users to download and store.