xarray allows several types for netcdf attributes. Is it expected ?

What is your issue?

Xarray is permissive regarding the type of the attributes. If using a wrong type, the error reveals the valid types: For serialization to netCDF files, its value must be of one of the following types: str, Number, ndarray, number, list, tuple

Using a non iterable type used to raise an Exception when reading the saved netcdf, but this is now solved with #7085

The pending question is whether it is valid to save netcdf attributes with type other than a string or not.
The following lines are working (in a notebook):

xr.DataArray([1, 2, 3], attrs={'units': 1}, name='x').to_netcdf("tmp.nc")
!ncdump tmp.nc

xr.DataArray([1, 2, 3], attrs={'units': np.nan}, name='x').to_netcdf("tmp.nc")
!ncdump tmp.nc

xr.DataArray([1, 2, 3], attrs={'units': ['xarray', 'is', 'very', 'permissive', ]}, name='x').to_netcdf("tmp.nc")
!ncdump tmp.nc

On the other hand, the following line raises an error:

xr.DataArray([1, 2, 3], attrs={'units': None}, name='x').to_netcdf("tmp.nc")

Start with the DataArray.to_netcdf examples in the issue and compare the behavior of numeric, NaN, list, and None attributes. Determine whether non-string netCDF attributes are intended to be supported; done should be a clear decision reflected in consistent behavior and an appropriate test or documentation update.