Serialize library provides interface SerializerInterface and multiple implementations:

• Json - default implementation. Uses PHP native json_encode/json_decode functions;
• Serialize - less secure than Json, but gives higher performance on big arrays. Uses PHP native serialize/unserialize functions, does not unserialize objects on PHP 7.

Using Serialize implementation directly is discouraged, always use SerializerInterface, using Serialize implementation may lead to security vulnerabilities.