the Three Conflicting Imperatives of RPGs

Fantasy RPG mechanics have to balance three imperatives: The Rule of Realism, the Rule of Cool and the Rule of Playability.
These are pretty self explantory but just in case:

Rule of Realism

The Rule of Realism means that the rules of physics, culture, and metaphysics in Scarterra needs to make sense enough that they don't challenge a player's suspension of disbelief. If I make up a rule explaining how Scarterra's laws of physics are not the same as Earth's laws of physics, then that rule needs to have a logical reason to exist and I need to apply every time unless there is a strong mitigating factor.
Example: Void Demons turn into piles of salt when they die. This is a universal law because Void Demons are born of Turoch's essence and Turoch's blood solidifies into salt. That means if you slay a Void Demon and it leaves behind a conventional corpse than either it's not truly dead or the thing you killed was not truly a Void Demon. I, as the Game Master cannot have a dead Void Demon corpse fall in a heavy heap as a corpse just once because I want the Void Demon corpse to trigger a pressure senstive trap.

Rule of Cool

The Rule of Cool exists that since Scarterra is part of the High Fantasy/Sword and Sorcery genre, we can relax our suspension of disbelief when it is entertaining to do so, provided it's not so ridiculous that it completely obliterates the Rule of Realism.
Example: The PCs are fighting a bunch of goblin zombies emerging from the water. Svetlana the half-orc's player wants to grab a zombie and swing it like a club to bash other zombies with it. Is this practical? Not really. Is this a more efficient way to kill zombies than Svetlana simply slashing zombies with her magic battle axe? No. But goblins are fairly small and Svetlana is very strong, so I'm going to let her try this maneuver and I'm going to let her do a fair bit of damage to both the zombie being used as a weapon and the zombie being used as the target because this is fun, and I want to reward players coming up with fun ideas.

Rule of Playability

The Rule of Playability states that the game should progress fairly smoothly and quickly. If a maneuver requires four or five rolls to accomplish, it should probably not be allowed even if it is both realistic and cool just because I don't want to slow down the sequence of combat more than is necessary. So if something either takes a long time to figure out or requires both Game Master and player to keep track of a bunch of nitpicky things, I'm not going to worry about it.
Example: As an casual HEMA enthusiast, I am well aware that weapon reach is a very important in melee combat. There are situations where having a longer reach is an advantage and there are situations where having a shorter reach is an advantage. Playtesting shows that keeping track of how one character has different reach relative to another character is a huge pain in the ass. Especially when there is a large group of opponents. Maybe this would be achievable with miniatures on a grid map, but my normal playing group is not a big fan of this style of play.
I assume that characters with short reach are trying to close the distance with their opponents and characters with long reach are trying to maintain their distance. Essentially all combatants are trying to maximize their advantage at all times, so rather than try to keep track of who has advantage on whom, I just assuming that some positioning moves are a "free action" and fold these into Dexterity + Melee rolls.

Tying this All Together

Is Legolas riding a shield like a surfboard down a flight of stairs while firing arrows realistic or practical? No. Is it cool? Yes.
Hypothetically if a character wants to do something similar in Scarterra d10, they can, as a nod to the Rule of Cool. As a nod to the Rule of Realism, the player must use their B-action to roll Dexterity + Athletics difficulty to not fall off the shield and roll Dexterity + Archery to aim shots, so success is not guaranteed, but you can certainly try to do this. As a nod to the Rule of Playability, you can figure out if your character succeeds or fails with only two dice rolls.

popular GIF from Avenger's movie by Marvel Studios and Internet memers

We build the plane while we are flying in it

If there is a conflict between these three Rules during gameplay, the Game Master will make a decision in the spur of the moment (so as not to slow things down overly much) and that rule will stand for the rest of the battle.
After the battle, or more likely after the session, the Game Master and the players can then discuss what worked and what didn't, and maybe make a new ruling for next time, or may the spur of the moment worked and we can solidify as "the normal rule" from then on out.
Two other addendums. First, the more experienced the players are, the more special rules we can include because the players understand the basics so well, but if there are newbie players involved than keeping things simple is probably for the best.
Second, there is an inverse relationship between game desirable game complexity and the number of players participating. In a solo game or a two-player game we can put in a lot of special modifiers for the Rule of Realism and the Rule of Cool, but if there are for or five players all patiently waiting for their turn to describe what their characters are going to do, it is probably best to keep things simple so the game can flow relatively quickly, pushing the Rule of Playability to the forefront.