Arrows and crossbows are excellent projectile weapons. The thing that made flintlock firearms so much better, was the weapons ability to puch through armor, ability to cause horrific damage, and its knockdown power.
1. A hot .50 cal or larger round ( The Hessians used a 58 cal round in some of their rifles during the American revolution ), lead ball impacts the body of the target at high speed ( Much faster than an arrow ). The ball flattens a little, penetrates the body, an proceeds to plow through tissue and organs. Bones are shattered as the hot round pushes through the body. Immediately, the body fights to combat the intrusion. Abdominal penetration was the worst, since the toxins in the targets bowels spilled into the body ( It could mean a slow, painful death )...A strike in the abdomen was almost always fatal. Limbs that were struck would most likely have been shattered, thus requiring amputation to save the target.
2. Infection was the next killer. LEAD, being very toxic, complicated healing, and intensified infection.
3. Muskets had good range, but poor accuracy ( A round projectile not being very aerodynamic or stable in flight ). This flaw was compensated for by " Massing fire ". It wasn't until the American Civil War that the problem was fixed by rifling the barrels and slightly elongating the round...Unfortunately the tactics did not keep up with the new technology.