The virtualization of the 5th Generation (5G) Radio Ac- cess Network (RAN), which distributes the Next Generation Node B (gNB) functions between a Central Unit (CU) and a Distributed Unit (DU), along with the emergence of slicing as a cornerstone of mobile communications networks, pose new challenges. One of these challenges is the management of the joint slicing and allocation of appropriate distribution of functions between CU and DU, known as functional split. In this work, a discussion on the challenges and open issues of joint slicing and functional splitting is posed, taking into account the additional complexity resulted from instantiating multiple slices per DU, each one with a different functional split. It is shown that the additional complexity, which is translated into higher cost, is worthwhile depending on the RAN deployment density.