Speed, Efficiency, and Accuracy
When a condition is met, the contract is immediately executed. Because smart contracts are digital and automated, there is no paperwork to process and no time spent reconciling errors that frequently occur when filling out forms manually.
Trust and Transparency
Due to the lack of involvement of a third party. Because participants share encrypted records of transactions, there is no need to question whether information has been altered for personal gain.
Security
Because the records of transactions on the blockchain are encrypted, they are very hard to hack. On a distributed ledger, each record is linked to the record before it and the record after it, so hackers would have to change the whole chain to change just one record.
Savings
Smart contracts remove the need for intermediaries to handle transactions and, by extension, their associated time delays and fees.
Difficulty to change
Changing the way a smart contract works is almost impossible because fixing a mistake in the code can take a long time and cost a lot of money.
Possibility of loopholes
The idea of good faith says that people will deal with each other honestly and not take advantage of a contract in an unethical way. But when smart contracts are used, it's hard to make sure that the terms are met the way they were agreed upon.
Third-party
Even though smart contracts try to get rid of the need for third parties, it is not possible to do so. Third parties take on roles that are different from what they do in traditional contracts. For example, lawyers won't be needed to write individual contracts, but they will be needed to help developers understand the terms so they can write codes for smart contracts.
Vague terms
Since contracts include terms that are not always understood, smart contracts are not always able to handle terms and conditions that are vague.