The arrow fires from the arrowhead when drawn back. If you look nearly straight down, you have to adjust your aim such that the show goes from the end of your bow to the spot you're aiming at. Most games have it fire from the back of the arrow, this one fires from the front. Once you learn how that interacts, you can learn to adjust. Takes some practice if you prefer not to mod the game. Technically speaking, you have to adjust this same way aiming anything but straight forward, it just doesn't affect the shot noticably until the angle is more extreme.

+1 for Slope Combat Assist. Hopefully they'll incorporate it into the base game, it only makes sense.

If it helps, it seems like the weapon hitboxes are pretty faithful to the animation when you attack.
So if you are performing a diagonal swing and want to hit something low down, you want to angle yourself so that the enemy is facing the side where your swing comes to its lowest point. If the lowest point of your swing is on your bottom left you want to face yourself a little to the right and vice versa.

Similar logic can also be applied when using your pickaxe to hit things either precisely or overhead.

Also, different weapon types have different combo's. When you hit your targets consecutively with the primary attack, you get different swings with your weapon - some even right up vertical !

If you:re not mod-averse, Slope Combat Assist mod does an excellent job of fixing this issue.

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.

Heavy attack with mace is the answer. Works on downhill at least. Uphill is a 50/50

You can leap and swing for uphill targets. The timing is a nit tricky. You can swing the pick against down hill targets. Bow even at melee range works for both.

You can solve this by using Iron Sledge. It has a wide damage area both horizontally and vertically.

Aiming up or down with bows in steep angles is a bit wonky, I don't think there's anything else to do about that except experiment and practicing so you can learn how to compensate to get the proper angle for various distances and targets.