(Glad to see others have posted answer, don't really have the time to give a detailed reply. Hope my questions where of some value.)
Here's a nice article covering major issues: Death and social media: what happens to your life online?
Main thing is your legal right to take actions on behalf of the deceased user's digital afterlife, and how delays in that might effect the outcome of your efforts. For example, if one of the reasons to access the accounts was to create a backup of the data and contacts, some email account service providers delete the data and/or account itself after a set amount of time. Another issue is as a result of this, someone might in fact acquire the email address after it expires to take over there identity; also unaware of any policies that allow usernames to be retired from reuse.
All and all, it's a very complex issue, but in the end, unlike to result in any threats. Main thing is the effect of having those accounts still online, and the impact seeing them might have on the ones they loved.