No concerns at all. One side is the legal requirements, the other side what your users really want to do.
There will be always users who leave your forum in fight. I do not delete their postings. They can delete their own data in their userprofile anyway excpet username and email address. I then will decide on a case by case basis, how to proceed if they request that, because theer are also legal arguments to keep the emails address for another 3 years.