Even if you put it exactly 6" away from the edge that balista covers at least 48" from that edge. That should be more than enough to be able to cover something important unless the entire battle literally just takes place on the other side of the battlefield (in which case your opponent is kind of an idiot for claiming it). Even with the "short" attack he'd still cover at least 30" from that edge which should be enough to provide covering fire for an objective or chokepoint or two... And that's assuming you stick it exactly 6" away from the edge, if you put it in a halfway decent position it'l definitly cover something of value with both attacks. It might not be the greatest of advantages, but it doesn't really have much of a disadvantage either. Assuming you have something with decent range there's very little reason not to garisson unless your opponent is turtling on the other side of the board. And that's assuming you place it in a terrible position. Assuming the RSE is placed in a "decent" position there's virtually no reason to garisson it if you get the chance. General downsides to all faction terrain, like an opponent potentially using it, would be interesting. I especially dislike the stuff like the Idoneth's boat, which doesn't just benefit nearby Idoneth, but also punishes nearby enemies. Thus making it pointless to controll for the opponent. Damned if you do, damned if you don't. However, I would say that every piece of faction terrain needs to at least start out under the appropriate faction's control. Fighting over it so you use it against the original faction is great. But potentially giving it to the opponent from the very start like our RSE feels rather bad. Which does give me an idea. What if an opponent had to break into the RSE if no seraphon have garissoned it yet and the doors are still "closed"? Waste a turn or 2 in combat breaking down the doors? That way at least there's some definitive cost to the opponent without having to rely on forced external choices like the arcane terrain swap, or having to put it in a far away corner to diminish it's value.