They are exceptionally vulnerable to cavalry, dire wolves and anything that is too fast for them to react. Since they can't shift before shooting, they can be outflanked quickly, unless you dedicate support units for their that point they are seriously expensive.

Starks have alot of ways to close in and wipe it out, lannister cavalry is also highly dangerous (namely The Mountain that Rides).

That said, i understand the point against the free folk, they don't have cavalry nor any fast monster. I can certainly see a unit of raider being almost wiped out by a volley of a scorpion. That being said, they should have the numbers to make up for it? Also, Giants are a serious threat too, even with the single shot mode.