issue_comments: 248255299

Or another way to put it: While typical metadata/attributes are only relevant if you eventually read them (which is where you will notice if they were lost on the way), units are different: They work silently behind the scene at all times, even if you do not explicitly look for them. You want an addition to fail if units don't match, without having to explicitly first test if the operands have units. So what should the ufunc_hook do if it finds two Variables that don't seem to carry units, raise an exception? Most probably not, as that would prevent to use xarray at the same time without units. So if the units are lost on the way, you might never notice, but end up with wrong data. To me, that is just not unlikely enough to happen given the damage it can do (e.g. the time it takes to find out what's going on once you realise you get wrong data).