Well, yes it is. But it is also a highly successful one if there are not many cancer cells present yet.
So, almost never.
Early diagnosis isn’t readily obtainable for most cancers, and even for those where it showed some real usefulness, what it has really improved is life expectancy after diagnosis, which is quite different from a reduction in actual overall mortality.
Many people won’t get checked for little, vague symptoms that may later be found to stem from cancer. Even when there’s some severe symptoms, a diagnosis may be severely delayed, especially in countries with no “free“ healthcare. At that point, chemo alone is practically useless. And even when it works, no doctor will ever dare declare you cancer-free, for the rest of your life. The disease can relapse after decades.