While a swirling melee is not optimal positioning for any dedicated archery unit, the Thorn Watch crossbowmen wield light arms and armor, and train extensively with longswords, allowing them to withstand direct assault which would scatter ordinary archers. Adept commanders will avoid such outcomes if possible, but can rest easy knowing that the Thorn Watch will not wither in the face of a surprise attack.
That said, their true strength is the crossbow, capable of puncturing even the heaviest armor. Proper positioning allows these troops to blunt even a vanguard's advance, and protect melee troops from archers or even cavalry.