If you have heard a story or two like that, such anecdotes are few, in opposition to a wealth of evidence about the benefits for diverse types of therapy and populations. In general, it is found that most relationships improve at least somewhat in response to counselling. Like any field of treatment that has a scientific basis but involves human variability, for example medicine, some cases improve tremendously, while others don’t change much, and some deteriorate. Of those relationships that do not survive, most would probably not make it anyway.
Aside from the normal variety of personal experience, these may be some reasons for those reports.
Couples therapy methods have improved over the decades, based on a combination of resarch and clinical expertise. Unlike 50 years ago, couples are no longer urged to vent their resentments by hitting each other with foam bats.
There is always some research in any field that has poor outcomes, but that often means that the modality under study is not so generalizable, not that it is ineffective for all.
A few critics who have a strong religious or political disposition in favor of marriage have complained that counselors are too tolerant, or even encouraging of decisions to split up. For such hard-liners, any breakup of a marriage under any circumstances is a bad outcome, not an option best left to the individual participants. The basis of my work is to help couples want to stay together, but counseling may sometimes lead participants to believe they are better off apart.
One prolific and unscrupulous internet advertiser (under a fake name similar to a renowned thinker in the field) has even unethically “exposed” and distorted critical reflections of another highly respected couples therapist, in order to promote an overpriced copycat e-book as an alternative! When he is called out on it, he swamps web search results for his fake name and the word “scam”, by publishing his own laudatory articles on numerous shill web sites.
Last but not least, any field of course has questionable practitioners who may have their own agendas, are not suitable for your needs, or are just incompetent. They might not be of great help, or might even give you biased advice when you are confused and vulnerable. For example, there has been some complaint that individually-oriented therapists are often not competent in couples counseling, due to inadequate training in that specialty, or may prioritize individual needs that are not likely met within the relationship.
It is a good idea to take care in who you select for help, and not to allow any counselor or therapist to make your most important decisions for you. Ask your counselor for a statement of their values to know where they stand. Shop around and interview several to find one who fits your style. But don’t let a concern about being taken advantage of stop you from getting help from a qualified professional.