Balance is generally desirable, as it means each option is viable, giving the player real choice. It also means the game is probably fairer, and there are no parts of the game that no one ever uses.
Balance is not that important.
Plenty of other things are more important.
Balance is also very easy to achieve. Almost anything can be balanced, which means that you can really do whatever you like, and just balance it all later.
Balance is not the same thing as fun. Do not excuse un-funness for balance reasons.
Chess is still a great game, even though white wins more than black. It's even less important that rooks are better than knights.
I know some cards I make are imbalanced (too strong, or too weak.) That's fine. It just happened that the simple, clean version of the card was imbalanced.
Choose elegance over balance.
As long as the card has some value, and isn't "feelbad" for anyone, I'm happy with it.
The other thing about cards is that a player has many of them. Some will be imbalanced individually, but a hand is likely to be balanced overall.
I do try to make all options balanced, but each option need only be balanced enough to be worthy of consideration by the player. If there are two options, and one is chosen only 25% of the time, that's still fine.
Someone recently asked me how I balanced Radlands. The real answer is that when balancing something, I just guess, and then play the game. If something is too strong, I make it weaker. If it's too weak, I make it stronger.
If I want something a bit more precise, I'll try to balance something by comparing it to something similar.
If you really want to balance, try to convert everything into your game's "lowest common denominator" resource, so you can roughly compare things.
Don't try to do mathematical calculations beyond that. I've been guilty of trying to balance things with a spreadsheet. It might seem doable, but what's the value of something that's slow, or the value of something that's overkill? There cannot be an absolute system for balance.
Be aware that something might actually be costing a second resource you're not aware of.
One action might cost twice as much as another, and give twice as much reward. However, both actions consume the player's turn, thereby making the bigger action better.
Some cards, in particular, are fair with a cost of zero, because they still cost a card.
A common mistake is to forget to compare the alternative. If there's a space where I can get two gold, and a space where I can spend three gold on a valuable item, what should the item's real value to the player be (if you had to convert it to gold)? Assume that gold and items are both good to have.
If you chose three, that's not balanced. Buying the item loses you three gold and gains you three, for a net gain of zero. The gold space gains you positive two gold. The item should in fact be worth five gold.
When you actually want to rebalance something, take note of the following:
A good designer balances by changing numbers. A bad designer adds rules text.
Make sure your cards (or other components) have a number on them. It could be a cost. It could be a points value. This allows easy fine-tuning of balance.
In Radlands, each player starts with three camp cards. These are important. When I started designing these camps, I knew balancing them would be difficult, so I added a numerical component. Every camp card has a number in its corner, and each player draws a starting hand equal to the total of these numbers on their camps. This way, weaker camps can be balanced by having a higher number, and vice versa.
Just make the simplest and nicest version of each object, and then balance it by tweaking its numbers.
Sometimes, you want to imbalance things.
In deck-builder game Dominion, you start the game with a deck of ten bad cards. You add good cards to it, and try to remove the bad ones. The Chapel card can remove four cards from the deck in one turn. This card is amazingly good, but the designer knew that. The idea is that a game with Chapel in it will be distinctly different from other games.
I like to imbalance things that are necessary for the game to progress, or otherwise remain healthy. Every option should be worthy of consideration, but I make these options have a clear edge over the others.
In an earlier version of my gangster game, location cards would come and go throughout the game. I wanted the board to keep changing. I could've said "every turn, or round, draw a new location card." However, I didn't want any kind of turn structure, where players had to remember to draw a new location. I wrangled with this for a long time, before settling on a fantastic and easy answer. I created a clearly imbalanced Getaway Car space, that people wanted to go to, but which also drew a new location card.
Don't forget that options should be similar in fun, as well as in strength.
In my gangster game, players can take various actions. Some
will increase their health. Staying alive is necessary, but it's also boring.
Players greatly prefer the actions that give them money, because money is cool —
it can be spent on all kinds of stuff. To make health more interesting, I
created some new and exciting places for the player to go, but only if they have
enough health.
In an earlier version, you would sometimes draw cards.
These were interesting, so I paired them with those boring health locations.
In the old Fighting Fantasy "Sorcery" game books, you could be a wizard, and cast all kinds of cool spells, or you could be a warrior, who had... two extra Skill points.
In Radlands, you have three water to spend each turn. However, there are almost no abilities that cost three water. It's a much more interesting choice to choose one of your abilities that costs two, plus one that costs one. One three-cost ability is usually a boring turn.
Balance is not always good.
In simpler, more fun-based games, imbalance can add excitement. It might not seem like a good idea for some things to be clearly better than others, but when you get them, it's exciting.
In Scrabble, some tiles are blatantly better than others. The blanks are the best tiles by a huge margin. That's imbalanced and a bit unfair, but there's excitement to be had every time you reach into the bag and draw your tiles.