Due to increased customization of products, manufacturing companies are subjected to assembly challenges arising from unique product configurations and higher number of variants to be able to produce in low volumes. This translates to an increased production flexibility requirement, while maintaining a high production efficiency. A higher variation among products would potentially disrupt the assembly line and create discrepancies in sequence, tools, and flow of operations. This paper explores how discrete event simulation (DES) can be used as a tool for assembly cell planning by applying key performance indicators (KPIs). A literature review is conducted to identify relevant KPIs that can be applied to compare different assembly scenarios. The KPIs are grouped under the three pillars of sustainability and analyzed as to how DES can be applied to assess them, thereby helping in evaluation and planning of an automation-assisted assembly cell. Finally, an interplay between Automation, KPI and DES, is presented.