First of all lets understand what is the composition of the weight gainers. The nutrition breakdown of a weight gainer is as follows:
10–25% of protein
5% of fat
70–85% of carbs .

If we drill down on the source of carbs, they are mostly sugars.

Let us categorize people who find it tough to gain weight:

Category 1 – Many do not eat enough food/ meals in a day and they imagine that they are eating a lot. Hence dont gain muscle/weight.
Category 2 – Many do not train hard and eat enough protein in their diet and hence do not gain muscle/weight.
Category 3 – Some people have very high metabolism and are unable to get in enough calories in their diet through whole foods.

The weight gain solution:

Category 1 needs to increase their food intake in general.
Category 2 Needs to improve their training and increase their total protein intake through food. A protein supplement may be handy.
Category 3 would find it beneficial to use a weight gainer shake once or twice a day to spike up their total caloric intake. And it would be beneficial for them to improve their performance and increase weight.

The weight gainer kind of supplements may actually make people falling under category 1 and 2 fatter due to the sugars in them.
Only people falling under category 3 would be really benefited from the weight gainer category of supplements. So understand what category you fall under and then choose your supplement wisely.