The obligators were involved because Straff was incompetent. Elend's army is organized enough in HoA to efficiently set up camp and set up a defensive perimeter quickly, and the only obligators Elend uses are scholars and scribes. Yes, the Scadrian soldiers are relatively inexperienced, but at least they don't act like the soldiers in WoK, who are supposed to be using some kind of phalanx and break formation before reaching the enemy.
The Alethi army uses short spears, probably only 4 to 5 feet in length, so the koloss will still reach them easily. Also, the thick, slippery, skin of the koloss will make it hard for the spearmen to stab them, since the spear will usually catch on the skin and slide to one side or the other. Archers are useless against koloss:
So the same coin attack that killed forty armored soldiers at Cett's keep "gets the attention" of koloss. And "hailstorms of arrows" does little to koloss except for killing a few of the smaller ones.
I think koloss will be just as effective against Roshar as they are against Scadrial