Your most unhappy customers are your greatest source of learning.Bill Gates
Although most will find an unhappy customer to be a pain point or stressor of some sort I always try to find the “silver-lining” in their cloud.
Be grateful for “unhappy customers” as they are the ones who care enough to actually tell you how they feel when they do not like something. This type of feedback is invaluable as they are taking the time to let you know something is not working. It might be something just not working for them, or it could be something affecting everyone using your product.
Granted, the “not working” can also be for any number of reasons ranging from utterly broken to functional misunderstandings although in all of these cases there will be something to be learned.
You might not have been aware something was broken, it could be an edge-case scenario that was not considered in testing. If there is a “functional misunderstanding” it could simply be a matter of improving and/or adding documentation to explain what the function does and what to expect when using it.
Taking the time to understand why a custom is unhappy will always provide a benefit even if it is nothing more than how to better address the feedback they are sharing.