Low-Power and Lossy networks are an integral part of the IoT ecosystem. These networks are defined by their shared features such as having limited resources and high occurrence of packet loss. A routing protocol for such networks called Routing Protocol for Low-Power and Lossy Networks (RPL) was proposed in 2012. Even though RPL is now standardized and well-accepted by the community, it still has areas to be improved such as load balancing, stability, and support for mobility. This study focuses particularly on approaches proposed for the load balancing problem in RPL. In the literature, many researchers aimed to tackle this problem by creating different routing metrics that handle different objectives. This review makes a thorough assessment of these works, their strengths and shortcomings, and provides future directions on the issue.