Benefits are, it feels healthy and loving? Like, it feels really deeply loving to me to allow my partner to do what they want. I don't have to suppress any romantic/sexual interest I naturally experience for other people when I'm im a relationship.
Problems are, it can be more complicated. If you have multiple intertwined relationships going on, you might need to have even more talks; more unpredictable things happen, more emotions to manage. In some cases it can be more total effort.