Balance is easy to recognize by a feeling of indecision. If you have several options that are more or less equally attractive, and they are still equally attractive if you are fully informed, that is a well-balanced design.
In the idle game Realm Grinder, you can get a bonus for watching an ad. They are not mutually exclusive, so you can keep watching ads, but the balance between them is interesting. During an event, one is a very slow stream of the event currency, which is nice. Most of the time, what you want is doubling your production for 4 hours. That is orders of magnitude better than the next option: +10% mana for 10 minutes. There are a few points in the game when that +10% is the difference between being able to complete a combo, but it effectively means that you get 1 minute’s worth of mana. Compared to doubling your production for 4 hours.
The last option is faction coins, which scales with your production. Ever so slightly. I clicked the button for an ad, and if I pick faction coins, I get 3500. I need 2.655*10^18 faction coins to get my next spell tier, and that is after using rewards to cut its cost. My costs are expressed in scientific notation, and the ad reward is 3500. Assuming 30 second ads, I would need to watch more than 721 million years of ads. Wait, no, there are 8 types of faction coins, and I need that many coins from two factions, so the number is a but below 3 billion years of ads.
Sometimes we say that an optional activity in a game is effectively required. These ads are very much not required.