Software-defined networking (SDN) is able to control and monitor the network through softwarization. This new emerging paradigm makes the process of placing controllers decisive to its scalability and performance. Several different initiatives have been proposed to address the controller’s placement in Wide Area Network (WAN) SDN using a variety of objectives, including enhancing the network reliability, latency minimization, load distribution among networks, and many more. Many valuable studies have found that the Controller Placement Problem (CPP) in the infancy stage of the research is a significant aspect of the WAN SDN with respect to splitting strategy. However, these studies have overlooked the network size of domains and network balance splitting. Hence, this study critically reviews and systematically classifies the recent literature on multi-controller scalability and various strategies of placing controllers in WAN SDN. In addition, this study concentrates on controller placement approaches in two main factors, mainly propagation latency and balanced partition as significant design metrics.