There are a number of things that can be seen as weaknesses which could hinder an interview with one’s grandparent. Let us look at second of them.
First, the grandparent might not really want to be interviewed for their life story. There could be at least two reasons for this. One reason is that the grandparent might feel self-conscious. They might feel like people should not be making a fuss about their lives. A second reason is that the grandparent might not like to be reminded of the fact that their life is nearing an end. When you go to interview your grandparents, it can feel to them as if you are implying that they are going to die soon and that you want to interview them while you still can. This is not necessarily something that the grandparent will appreciate.
Second, you might not be willing to ask them all of the questions that a good interviewer would ask. For example, there are traumatic experiences in many people’s lives that have truly been important in shaping those lives. Those experiences are relevant, but you, as the grandchild, might not feel comfortable asking about those things. You might not feel like you can really intrude on your grandparent’s privacy in that way.
Either of these would be a serious weakness that could hinder the process of interviewing a grandparent.