One of the great strengths of the 6d6 system is the ease with which ideas and cards can flow from one setting to another. It is simple to have pirates with flintlocks and cutlasses fighting space ninjas with plasma cannons. However, that is likely to end with pirates smeared all over the poop deck.
Cards between different settings are compatible but they are not necessarily balanced. A card that realistically represents a 17th century firearm will be outclassed by a realistic card for a modern weapon because there are good reasons why modern armies don't use Napoleonic era technology. To design a setting where characters with flintlocks and characters with plasma cannons can be equally effective, some compromises have to be made. Advantages and disadvantages have to be balanced out.
Because of the trade-off needed to create cards within any setting, cards will almost always be out of balance when taken out of that setting. The core cards, those that represents real life abilities and equipment, are passably realistic. Some compromises have been made to ensure balance but they should be compatible with any other setting that is broadly realistic. Cards from specific settings, such as 6d6 Magic, are less likely to be balanced.
Balance itself is a nebulous concept. It is not something that can be worked out with an equation. Subtle changes to the wording of card titles or descriptions can cause vast changes in the effectiveness of a card. The only way to assess whether a card is balanced for a setting is to repeatedly try it in play tests. The 6d6 RPG is not the product of a careful calculation. It is the result of hundreds of hours of play tests that have worn down the rough edges and slowly brought the system into balance.
A Final Note on Balance
Not everything is created equal. A nuclear bomb is more powerful than a hand-grenade and always will be. Certain combinations of abilities and equipment are more powerful than others.
The aim of balance is not to make everything the same. That approach creates a very dull game that makes a player's choices meaningless. Instead, balance is about the trade-off between the cost and its reward. The harder it is for a character to acquire and deploy an ability, the greater the reward.