Power BI Desktop imports M queries, data model tables, DAX measures and KPIs, and any power view for Excel sheets.
Workbooks with significant dependencies on items not imported such as Excel formulas, standard Excel tables (not model tables), worksheet range data, standard Excel charts and conditional formatting may need to remain supported in some capacity. For example, a minimum amount of data could be imported to Excel's data model to continue to drive Excel-based reports and this workbook could be uploaded to the Power BI Service and refreshed.