Melissa Coates goes into exactly what it is that a Power BI admin can see and do:
I wrote about (and updated) this topic previously, but this is so important that it warrants revisiting. So let’s have a quick chat about what privileges a Power BI administrator has with respect to accessing data throughout the Power BI tenant.
All metadata throughout the tenant is available to the Power BI administrator (ex: if they want to enumerate a list of workspaces, reports, dashboards, etc using the APIs). So, metadata is easily discoverable but — technically speaking — a Power BI administrator cannot access datasets in Power BI unless they have permission to that workspace. However…
Read the whole thing.