There's three variants: Focused, Spread and Blast (though that may get a new name, what with the new Blast element).
Focused has a small hitbox in which the hits drop down. The hitbox is about the size of your character, maybe slightly bigger. It's ideal to hit specific weak spots that are hard to reach, like Duramboros' humps.
Spread is similar to Focused, except the area of effect is way larger. Because the area is larger, it's less effective at hitting specific targets and has a reduced damage output because of this.
Finally you have Blast, which delivers a single, explosive hit at the target location. It's fairly powerful because it's a single hit compared to the multiple hits of the other two, but the arrow needs to hit the ground before it explodes, reducing its use against flying opponents. The explosion looks similar to a Large Barrel Bomb, with a similar area of effect.
As you might've concluded from the descriptions, Spread and Blast have the potential to disrupt people a lot.
Getting hit by a Spread arrow rain is similar to getting hit by Pellet S, flinching a hunter who's not using an attack with Super Armor. You can probably imagine what will happen if this is spammed with no regards to your teammates.
Blast literally blows allies away like a LBB. No damage is dealt (IIRC), but people will be sent flying, even if they have Super Armor at the moment.
Focused Arrow Rain can still disrupt teammates like the other two, but it's less likely to happen due to its small area of effect.
To be honest, I never use the Arrow Rain, Rapid shots to weak spots (Zinogre's head/horns, Alatreon's head) or Pierce through long monsters (Jho lines itself up perfectly, same for Plesioth) has always felt much more satisfying.
I haven't seen any damage calculations for the bow (and doing said calculations must be utter hell, what with taking charge speed, charge level, range and amount of arrows that hit in mind), so I can't say if the rain is better or worse than normal attack.
Find a play style that you're comfortable with, but keep an eye on your allies.