One potential edge case is if someone directly calls a ufunc reduce method, e.g., np.add.reduce. Previously, this would cast the xarray object to a numpy array, but now it will raise an error.

Example: ```

current xarray

In [3]: np.add.reduce(xr.DataArray([1])) Out[3]: 1

with this pull request

In [3]: np.add.reduce(xr.DataArray(0)) NotImplementedError: reduce method for ufunc <ufunc 'add'> is not implemented on xarray objects, which currently only support the call method. ```

Note that the more commonly used aliases for these reduce methods, e.g., np.sum() will continue to work since they check for sum() method on their argument.

There are also a few other ufunc methods that get used occasionally.

I think I'm OK breaking these because usage is so rare (and the work-around of casting to numpy arrays is so easy) but this should probably be noted in the release notes.