This API looks at each end point item (reports, dashboards, datasets, datamarts etc), i believe, and resolves the users/permissions for each associated dataset being consumed by the end point. This is extremely valuable for dataset,report and workspace permissions and licensing management across the tenancy. Unfortunately the 200 per hour means it is not a viable option, currently, for anything but small tenancies.
We can get the resolved users from each endpoint independently via specific API GetXXUsersAsAdmin.
The DatasetUsers API does not have a feature to get only direct Dataset assigned users (dataset access shared with user). Its all or nothing.
If you would like to suggest feature improvements, you may vote the idea and comment here to improve this feature. It is a place for customers provide feedback about Microsoft Office products . What’s more, if a feedback is high voted there by other customers, it will be promising that Microsoft Product Team will take it into consideration when designing the next version in the future.