I would expect similar toxicity issues between mxe and ket. I have a feeling, however, it's mediated by the amount of chemical you're taking in. As doses with methoxetamine will tend to be smaller due to its potency, so will the risk of toxicity.
Methoxetamine is unknown in the scientific literature, however. I don't think LD50 studies have even happened; dosing mxe is playing with fire.