This guide covers known issues and limitations with using the Supermetrics API connections to PowerBI.
For some queries, Power BI was only able to process 20000-30000 rows without running into issues. There's no clear reason why this is happening, it would just sometimes timeout during the processing if the rows amount was over 20000. The issue has been reported to Microsoft to see if this can be addressed.
To work around the problem, you can create multiple queries to cover the full range of data, splitting them up by date.
PowerBI can refresh web connectors, like the Supermetrics API, when scheduled to the server. However, there are issues with the scheduling of PowerBI reports to refresh automatically which are caused by the process the tool takes to refresh the data. First it access to the web page is checked for the web connector, and then the data is retrieved. This first step though can fail, which can cause the refresh to fail.
To work around the problem, set the output type to JSON. More clicks may be needed but it should certainly work without the possible access failure.