To me, Minecraft is not really a sandbox, because it does have clear goals and progression.
Now that this is out off the way, balance in single player games (no matter the type, sandbox, rpg, puzzle...) is a set of rules that are placed on the game. Usually, those rules come with associated rewards, and the whole game is made so that the player enjoys the game.
If the game is felt "too easy" by someone, he/she might not enjoy it as much. The same is also valid for "too hard".
But these "too easy" and "too hard" are entirely subjective definitions, someone good at plateformers can find Meat Boy easy, where others that are not used to having to be really precise will find it hard quite early.
I choose an example that is not Minecraft, because the lines are easier to see, but it's the same here.
Remember, when you first started playing? There are good chances that you were scared of the first nights, so much so that there were a lot of guides on how to survive it. For many people, it was even a goal on a checklist, to have a safe haven when the sun sets down for the first time.
Was it that the first few nights were too hard?
And what about caving without armour, always looking out for skeletons and creepers?
Experienced players will have set strategies to deal with such things, making surviving the first night reall an easy thing to do, even without bothering about hiding. Does it means that the game is too easy?
As it was rightfully said above, the "right" balance in single player is what is felt like such by the player. Different people will feel that some things are too easy, some others too hard, and play accordingly, in order to enjoy their time.
In a multi-player environment, it's a bit more complex, because you need to think about the enjoyment of everyone, not just of one or a sub-group of people.
Usually, it goes either to a majority, or to the ones actually paying and/or maintaining the servers, because they are the ones investing time or money in order for them and others to enjoy their time.
If you don't enjoy your time, you might want to change how you are playing, either changing the configuration files when available, or changing what mods you are playing with.
I said it before, and I'll probably say it again, but it is so easy to install, uninstall and update mods now, even more with tools like MultiMC. Of course, you need to work a bit on the configs to have all IDs not conflict.
FTB, as well as other mod packs, present you with a "ready to play" package, but that comes with the cost of it being tuned around what their creators think is a good balance.
If you do not agree, nothing prevents you from either changing things around, or to even make your own brew.
Constantly complaining about "balance" only works in three cases.
When the balance is set so that part of the players get an unfair advantage over others (not counting the hardware part, of course), in a competitive environment.
When a very large proportion of the community gets together against a specific problem.
When the "creators" of the content (game designers, modders, pack makers...) are willing to sacrifice their vision of how it should be.
At least, that is how I see it.
The thing is, modders will often make changes, but it is often in the light of some uses of their mod, and the changes usually go toward what they want from their mod.
Some mods try to balance against others, some go as far as changing things out of their mods, and some decide that there are way too many combinations out there, and balancing for one case would potentially loose them the players that are interested in other combinations.
... Sorry, I did it again, long post with not that much content, and a lot of personal opinions ...
