Azure AD B2C is a serverless CIAM (Customer Identity and Access Management) offering from Microsoft Azure that provides authentication and authorization for users. It has limitations such as inflexible user interface, high SMS pricing, and inability to backup or export user data like password hashes, which may lead customers to consider migrating to another solution. The decision to migrate depends on the specific needs of the organization, with bulk migration being a more complex operation requiring users to reset their passwords, while slow migration allows users to continue logging in without resetting their passwords. Both approaches have pros and cons, with bulk migration providing a single source of truth for customer data but requiring engineering effort, and slow migration having less impact on users but taking longer and potentially affecting systems and employees.