X Gon' Give It To Ya [1]
It hits so many receptor sites and interacts with so many other drugs in the brain/liver, ketamine is not a dirty drug in that regard. I think this puts restrictions on its comparative therapeutic use. [1]
Ketamine however is pretty hard on the kidneys, and its LD50 is significantly closer to therapeutic levels than DXM. Some quick searching shows it to be about 20x the dose used for depression while DXM, near as I can tell checking out a few sources that didn't always agree, has an LD50 roughly 100x higher than therapeutic doses. That doesn't mean it can't do other bad things to you (it can) but it's got a much more forgiving margin of error on acute toxicity. Most literature on severe overdoses mentions polydrug usage.