I think it is likely more efficient. That said, cows do have the advantage that the food they consume needs little to no processing in order to produce milk. The yeast needs pretty precise processing of the incoming mash both to make sure a wild yeast strain doesn't make it's way in, and to make sure the yeast ultimately produces the right proteins.

You can't just throw in grass clippings into a vat and get whey. You can throw grass clippings into a cow to get milk (though, TBF, I dislike grassy milk).

I agree it’s likely to be more labor intensive per lb of feedstock, but only 21% of calories in milk are protein and overall milk has ~10% of the initial energy. So you’re looking at ~2% of the energy from these crops ending up as milk protein.

That’s a lot of room for improvement which then means far less labor on growing crops.