Mastering Blockchain Programming with Solidity

Chapter 2 

1. Both of these functions are available with the address data type. The transfer call fails even when an internal function call throws an error; however, the send function returns the bool type as false in the event of a failure.
2. If you use the send function in your contract, then you would need to explicitly check for the returned bool type, which could be missed, which would lead to a higher chance of getting it wrong. Even if there is no more gas left during the send call and an out-of-gas exception is thrown, it will return false as a result. For these reasons, it is recommended that you use transfer rather than the send function.
3. It is not recommended that you use the delegatecall function. This functions use low-level calls. ...