Well shield walls provide cover so longbows have less effect.
Magic is both a blessing and a curse. Magic gives things like wall and pit spells, as well as healing and buffs. It also makes things like tornadoes fireballs, and 80 ft area thorny entanglement.
All military formations are made to fulfill some goal. As a defensive formation, specially in a choke point I can see the pike formation being extremely dificult to fight.
Overwhelming numbers has always been a staple in combat, it's why the rule is generally 3 time or bigger numbers for an assaulting army. In the case of normal infantry vs pikemen, its effectively a large scale version of reach tactics, the goal being to have people not move closer and punish those that do. In pathfinder, my guess would be that all those people have Combat Reflexes and a large dex bonus.
If each has Dex 18, that's 5 AoO each person with 20 people in a formation for 100 AoO, on top of the full attack vs anything in the attack area. If they have a class or ability to attack or threaten with the haft, it gets worse for anything approaching. The gunslingers would deal with tanky units that get close.
