A basic limitation in Direct Discovery is that it requires a SQL compliant data source. A custom connector that can translate SQL to the data source' query language can be used i
f the data source is not natively SQL compliant. From SAP connector perspective this means that only the SAP SQL Connector can used to populate a Direct Table. This is also specified in the Direct Discovery technical documentation. There is no current plan to change this limitation, since it relates to the way the delta load request needs to be structured. QlikView 11.20 Direct Discovery: Technical documentation
An other aspect to consider with Direct Discovery is that the data source needs to be responsive, so that the end-user gets a good experience. Direct Discovery will always add latency to the application updates, when data is extracted and transferred from the database side to QlikView. The recommendation is to evaluate if the application requirements can be fulfilled without using Direct Discovery, so that the end-user gets best possible experience. Common solutions is to split the application into smaller pieces so that it will fit in memory, or to upgrade hardware so that larger applications fit in memory.