Yeah, I had considered the endless afk farming as a potential downside. The hope would be that the ambushes would find other places on the map to appear so that instead of approaching the player, the players still would have to move around to find and eliminate the enemies, but the enemies could get steadily stronger.
Many thanks for everyone's input! I'll try out the bosses and defense objectives and see if I can't get a feel for what I'm looking for there.