I've learned that one of the complications of ECT is the possibility of triggering a prolonged seizure or series of seizures that can last for many minutes during which the patient might be unable to breathe. Currently, doctors can prevent this by administering oxygen and using anticonvulsants to arrest a seizure that continues for too long. Neither of these options is available in a Victorian-based setting in which there were no effective treatments for seizures or coma.
Some can become psychotic which can be accompanied by hallucinations. I don't know if they could have hallucinations WITHOUT psychosis. But I don't think anyone would argue the point since strange things happen in the brain with electricity especially in the setting as the early years of ECT. I’m sure hallucinations could also happen after the hypoxia and coma.