One of the things that put me off when I came back to FreeBSD about four years ago was that the keyboard wouldn't work when the login page popped up. The reason for this was that slim and lightdm were both enabled. One was taking the input, the other was taking the display. Took a while to find but it is one of those things that makes a naive user like me think it is system instability.
Also especially with dbus running, one DM could perceivably try to consume events destined for the other.
Also especially with dbus running, one DM could perceivably try to consume events destined for the other.