Undercover work can be extremely dangerous, not only physically, but even mentally. Though it's not often, some UC's start believing they are the person they're playing. They might get too focused on their roles, and it could cause a lot of psychological damage to be undercover for so long. Also, there are physical risks of being undercover, especially if said cover is blown.
The FRCP 26(a)(2)(B) is a requirement for the expert witness. If the expert fails to include this, then there is a high chance that the testimony can be thrown out.
