Keras is pretty good unless you hit some custom loss function that needs to do operations that aren't defined in Keras' backend, then you suddenly have to switc

Keras is pretty good unless you hit some custom loss function that needs to do operations that aren't defined in Keras' backend, then you suddenly have to switch over to write them in TensorFlow with some ugly consequences (sometimes you don't know which operations will be GPU-accelerated; slicing vectors to compute and aggregate partial loss functions with some complicated math formulas might force computation onto a CPU).