Back when we were using TensorFlow, whenever we wanted to try something new, sooner or later we would find ourselves wrestling with its computational graph abstraction, which is non-intuitive, especially for models with more complex control flow.
That said, we are keeping an eye on Swift + MLIR + TensorFlow. We think it could unseat PyTorch for R&D and eventually, production, due to (a) the promise of automatic creation of high-performance GPU/TPU kernels without hassle, (b) Swift's easy learning curve, and (c) Swift's fast performance and type safety. Jeremy Howard has a good post about this: https://www.fast.ai/2019/03/06/fastai-swift/