Skirmishers have no formation of ranks and files and so lose some of the normal bonuses that apply to such units.
Skirmishers receive no combat bonus for additional ranks in their formation.
Skirmishing units have no flanks or rear in combat. This means the enemy cannot receive a bonus for fighting to their flanks or rear. Similarly the unit cannot be charged in the flank or rear whilst engaged in combat, so no Panic test is required for this.