I don't do much ML (this kind of ML at least), so I know I'm not the target audience for these libraries. But I'd wish they were written in some language with static typing for IDE help. The API interface/tweaks-to-be-done for some of them is enormous, and mostly undiscoverable.
I mean, just looking at the "getting started, 30 seconds to Keras"[0], there are so many magic strings and options. Of course, if one is well-versed in this domain, they make sense. But it's hard to grasp, and Keras is supposed to be the high-level one.
[0]: https://keras.io/