For bows and other ranged weapons you need to take into account the point from which the arrow is loose. There are mods that "fix" the targeting somewhat but not the loose point.

Attacking up/down is a problem because most projectiles start quite far ahead of the player, usually around the tip of the projectile when still in hand. So the bigger angle, closer the target and longer the weapon you need more adjustment. It sucks but just think you are standing a meter or two more forward than you actually do.


As for hitting targets with melee. Each weapon swing has different hitbox. Some attacks go very low some can go quite high. Daggers for example can hit things that are quite low. In general Spears are kings when it comes to height. They hit decently high and are easy to use when jumping unlike most other weapons. And when something is too low, like leeches or mobs on lower steps/rocks, you just throw them at your feet and retrieve instantly.

I wou'd not expect this to be "fixed" (you can try mods) as terrain plays a major factor in combat mechanics, and as many other things in the game is a challenge to be overcomed. (plus there are weapons like sledge or mistlands stuff that make it less of a problem)
Thou personally i'd love to see the projectile targeting fixed... maybe some day.