The information I've seen regarding deep discharge life-cycle for sodium ion is that the latest tech is actually extremely good, at least according to this. I don't see how the lower voltage is a problem, since for grid situations you'll have step-up transformers anyway, and the batteries can just be hooked up in series to increase the voltage.

They use abundant materials, will be much cheaper than lithium ion, don't need to be actively cooled, and massively lessen the risk of rupture and fires.

The low density per unit of weight isn't relevant for grid storage, so they seem pretty ideal.