When a user with specific row level security consumes a report with Top N, it doesn't show HIS/HER specific Top N. This person might not have access to those Top N's and the report will be blank or only show a few items.
So, if you look at a top 20 report at a limited roll, you may only see 5 items instead of your specific top 20. It's worse when you slice by month - "Items" drop off because they may not be top N. NOT good, basically negates using Top N for anyone with RLS at least from SSAS Tabular. Not sure if it effects everyone.
Also, Top N needs to work at multiple levels in a table or chart.