...movements are ideal.

A 2-handed underhanded toss is much easier to replicate consistently than an overhanded shot.

Add that fact to the fact that soft underhanded shots will get friendlier bounces on the rim, and I think it's impossible to argue that underhanded wouldn't be better for almost every shooter, no matter what level.

Next time you're at the church, shoot 100 free throws overhanded, and then 100 free throws underhand, and I can almost promise you you'll make more underhanded.