I would say that compared to Greykite, Darts really attempts to unify a wide variety of forecasting models under a common simple and user-friendly API. There are many differences, but for instance, AFAIK there's no deep learning model in Greykite (it focuses on two algorithms: their built-in algorithm and Prophet), whereas Darts tries to lower the barrier for using deep learning models for forecasting. Crucially for ML-based models, it also means being able to train on multiple (possibly thousands or more) of possibly multi-dimensional time series.