When to use a new field on the user account or a custom table and when use user.data service?
The purpose of the UserData API is to allow the storage of certain pieces of information related to a particular user. Its concept is similar to the State API in that the type of information stored is not a configuration that should be exported. In other words, it is specific to the current environment (but belonging to a given user rather than a system or
subsystem).

Users are content entities, who can have fields of various data types. These fields are typically used for structured information pertaining to the user, for example, a first and a last name. However, if you need to store something more irregular, such as user preferences or flag that a given user has done something, the UserData is a good place to do that. This is because the information is either not something structured or is not meant for the users themselves to manage.

In the other hand if you want to be able to export and import your user information in different environments then you should use fields.