it just seems that the finger release for overhead casts, as u say, you can overshoot or undershoot. sort of simple. but sidearm, a slightly mistimed finger release (for me) can result in casting way to the left or right, or actually whichever direction the arc of your cast is taking... i just feel a lot more can go wrong, that's all, with the sidearm.
i agree with others here about muscle memory, etc... the eye looking at the target - yes, that seems to result in best accuracy. but then there's dodging the tree or cliff next to you, or the little opening in the trees you're casting thru - i'm focusing on that...not exactly where my cast will end up. so, much less accurate, for me. but it gets my bait where i want it.
the other thing is, i'm using two rods, one five foot, one six foot.. they just seem so different... same 4# line, but the lures might be 1/16 oz vs 1/32 oz... more variables...
but ultralight spin casting is a total hoot! i miss bait casting, but that's, for me, 3/8 oz weights on up.
uh - you can roll cast, like with a bait caster, using ultralight? i don't no how to roll cast a bait caster - i had to use youtube to see what it was... and i'm just saying sidearms seem like, well, nfl passing, vs overhand, which is college... i normally sidearm, but yeah, i've got to be focused doing it, if i need accuracy.
i'm still working on my feathering --- it's basically my 'oh ****' helper.