>I guess balance in single player is about difficulty?
In a sense balance is always about difficulty. But it's also about not ruining all the depth your game offers. If one of the easiest paths becomes objectively the best to achieve a goal, there's no reason for the player to explore or use any of the other paths.
This appears in all kinds of systems: if there's one class that's just the best, your class system is unbalanced and most players will barely play the other classes. If there's this "one simple trick" to earn lots of money (e.g. mine that one very common ore, like in No Man's Sky), an interesting economy becomes a boring timesink due to being unbalanced. If one of the early weapons is the best, everyone will use it and all the time you spent designing other weapons was just a waste.
One of the things that I'm going to do in my game, is to make entire categories of items unusable or gimped across entire bands of space. So as you progress (the leaderboards are mainly based on how far in "the direction of difficulty" you can get) you will eventually get to a point where you have to abandon your favorite strategy and come up with a new one.
Even without this, due to the rarity of items that have little penalty for benefit, you will often have to improvise, as your required item will likely not be available with exactly the right combination of compatibility and stats.
In a sense balance is always about difficulty. But it's also about not ruining all the depth your game offers. If one of the easiest paths becomes objectively the best to achieve a goal, there's no reason for the player to explore or use any of the other paths.
This appears in all kinds of systems: if there's one class that's just the best, your class system is unbalanced and most players will barely play the other classes. If there's this "one simple trick" to earn lots of money (e.g. mine that one very common ore, like in No Man's Sky), an interesting economy becomes a boring timesink due to being unbalanced. If one of the early weapons is the best, everyone will use it and all the time you spent designing other weapons was just a waste.