It's not that they are bad per se, it's just that it's a huge waste of money, since you can make them all by yourself by mixing a pure protein powder with extremely cheap dextrose powder.
Plus, there are more efficient ways to increase your caloric intake: Ground flax seed in milk, olive oil in shakes, nuts and everything containing some good fat. Usually weight gainers don't contain much fat so they almost never give more than 400 kcal per 100gr. You can make some healthy, liquid caloric bombs by yourself:
500gr whole milk
50gr chocolate protein powder
50gr honey
25gr olive oil
300gr bananas
This should be about 1000 kcals (70gr protein, 90gr carbs, 40gr fat) and you could drink half of it in the morning and the other in the evening... I could come up with lots of delicious shakes...