As of mid-December 2020, we have enabled the feature to use the "Statistics Finder" endpoint (aka "double-pivot") in the LinkedIn Ads API, which will now permit combinations of dimensions that were not possible before. Up to three from different pivots/report types.
However, there are still some limitations and quirks that may impact what users are able to request. These are described below.
Conversion Dimensions Only Return Data for Conversion Metrics
Using conversion-specific metrics will work fine with conversion dimensions (i.e., Conversion name, Conversion type, etc.). However, if you use other metrics like Impressions, Clicks, etc. with the the conversion dimensions, they will return empty.
This is an expected behavior as the other metrics cannot be split or attributed to the conversions themselves, and are not supported by the API.
Demographic and Forms Dimensions are not Supported in Statistics Finder
Sadly, the demographic dimensions (Member industry, Member country, etc.) and the ad form dimensions are not included in the "Statistics Finder" endpoint, and thus can only be queried alone. (See API reference here for what is permitted.)
If you try to combine demographic or forms dimensions with any other dimension type, you will get an incompatible field error.
We realize being able to combine the demographics dimensions with the others was one of the key requested use cases, but the API does not support it still.