I do agree with this and think going for more niche topics can be good and probably interesting for an interviewer, but you absolutely need to make sure you know about it inside and out. I say this only because it could be that the interviewer is an authority on this and can therefore really challenge you or, indeed, they may not know as much and so might drill down into the topic in seeking you to explain your understanding more fully. I also find it is good to pick something you genuinely find interesting as opposed to discussing something that you do not. You are far more likely to be motivated to research a topic of genuine interest to the level you will need to do to go into an interview and be able to answer anything on it.
when picking a commercial story, would you say it’s okay even if it only links to a small area of practice for the firm and not one of their key practices?