I think that there are situations where these questions could have good answers. Specifically the F-104 comes to mind, as it had an unusually high accident rate. It, was considered a difficult airplane to fly, and a question about the reasons behind this would seem to be on topic and not too broad. There are other cases where incidents can be traced back to design or maintenance issues. If the question is specific enough to address this, it may not be too broad.
On the other hand, you are correct in your observation that most general cases are much harder to answer. The regulation of the aviation industry makes it difficult for a certain airplane to be significantly more dangerous. If a repairable flaw is known, the repair is mandated. Otherwise, the airplane is grounded. Because of this, historical questions may be more likely to have good answers.
So I don't think the whole category of questions should be off limits. If the question is specific enough, it could have a good answer.