Hi Will,
We spent a *long time* sorting out the messy details of __array_ufunc__ [1], especially for handling interactions between different types, e.g., between numpy arrays, non-numpy array-like objects, builtin Python objects, objects that override arithmetic to act in non-numpy-like ways, and of course subclasses of all the above.
We hope that we have it right this time, but as we wrote in the NumPy 1.13 release notes "The API is provisional, we do not yet guarantee backward compatibility as modifications may be made pending feedback." That said, let's give it a try!
If any changes are necessary, I expect it would likely relate to how we handle interactions between different types. That's where we spent the majority of the design effort, but debate is a poor substitute for experience. I would be very surprised if the basic cases (one argument or two arguments of the same type) need any changes.
Best,
Stephan