There are absolutely no restrictions on how you assign custom users abilities.

According to the Salesforce documentation, the answer is false. There are some restrictions on how you assign custom users abilities. A custom user is a user who has a custom role assigned to them in Marketing Cloud Account Engagement. A custom role is a feature that allows users to create and assign custom permissions and access levels to other users in Marketing Cloud Account Engagement. A custom role can be created and managed by the administrator in the Admin tab in Marketing Cloud Account Engagement, and it can be assigned to new or existing users. A custom role can be assigned to a user by using different methods, such as editing an existing default role, assigning new users during the import process, or adding a batch of existing users using table actions. However, there are some restrictions on how you assign custom users abilities, such as:

You cannot assign a custom user the ability to create or edit other users, roles, or connectors. This ability is reserved for the administrator only, and it cannot be delegated to a custom user.

You cannot assign a custom user the ability to access or edit the data or settings of another user, unless the custom user is the manager of that user. A manager is a user who is responsible for overseeing and managing other users, and who can view and edit their data or settings. A manager can be assigned to a user in the user record, and they can have a custom role or a default role.

You cannot assign a custom user the ability to access or edit the data or settings of a prospect, unless the custom user is the owner or the assigned user of that prospect. An owner is a user who created the prospect in Marketing Cloud Account Engagement, and who can view and edit their data or settings. An assigned user is a user who is assigned to the prospect in Marketing Cloud Account Engagement, and who can view and edit their data or settings. An owner or an assigned user can be assigned to a prospect in the prospect record, and they can have a custom role or a default role.