Hypoxic (low oxygen level) training, as most of the things we do, is good if practiced in moderation. There are of course a lot of debates on how much is ok to increase performance, without pushing the limit in damaging your brain.
When preparing for an above 8,000 m expedition, of course you think about how to best prepare for the extreme low level of oxygen, especially for the Death zone, where it is said that the body cannot acclimatize. The idea of training in low oxygen environments (created artificially in special rooms, tents or with a special mask) is so that your body will learn to create the same amount of energy with lower amounts of oxygen. This happens through changes in your body, that include increases in erythrocyte volume, maximal aerobic exercise capacity, capillary density and economy. The downsize of this training is a muscle mass reduction, diminished power outputs, slower post training recovery times and brain cells irreversible damage.
The worst case scenario when climbing at extreme altitude is the high-altitude cerebral edema (HACE), which can happen without proper acclimatization and fitness level. A climber severely affected by extreme altitude may experience amnesia, confusion, delusions, emotional disturbance, personality changes and loss of consciousness.
Personally, I have chosen not to do any artificial hypoxic training as the negative outcomes are too risky. What I did though is prepare at high and very high altitude (such as climbing Mt. Kilimanjaro twice this year, and Mont Blanc, close to the expedition date), combined with a rigid and tough training program. These expeditions helped me increase the amount of red blood cells (the cells that grab oxygen), which I believe will significantly help me in my upcoming Manaslu expedition at 8,163 m, where I aim to climb without using supplemental oxygen.