What issues can arise from using an alternative to gdm with gnome-shell?

view full story

http://askubuntu.com – This question is in context of, the computer in question is used by several people, some use Gnome-Shell, some use Unity, and the two are designed to work with different Display Managers (GDM and LightDM respectively). I have read that using LightDM you won't get the Gnome-Shell lock screen, but what other issues are you likely to hit when not using GDM? (HowTos)