When would you not use Looker Studio?

When you need a governed semantic layer so that a metric means the same thing everywhere — that is what Looker or a modelled Power BI dataset provides.

When you need heavy transformation. Looker Studio has no equivalent of Power Query or dbt; data should arrive already shaped.

When performance requirements are strict against large raw datasets, or when you need enterprise features like fine-grained row-level security defined in the reporting layer rather than delegated to the source.