If it's done, every three years, or so, from when boiler is new, it's fairly effective. If loose sediment is left, for years, however, it will go almost as solid as lumps of concrete. Then, a power flush can't hope to be effective. It may be possible that a chemical treatment, followed a couple of weeks later by power flush, may help. As mentioned above, you need to be aware that it will tend to "find" any minor leaks, and make them worse.