Funny enough this was actually an issue in the earlier days of a hardcore bigworld MMO I used to play called Mortal Online. Reds (gankers) would hang out near the boundaries and pop in out of nowhere. This was largely more of an engine limitation of UE3 as I understand it, and was eventually mostly worked around. Now many of the UE3 painpoints are softened with UE4 they have started Mortal Online II.