The success rate is very good if your lower than normal EF is the result of a dilated left ventricle. If there is heart muscle damage due to heart attack, virus, drugs alcohol, etc. the return of normal left ventricle functionality is not very good.
A dilated LV can occur from an overworked heart. The overwork can be from high blood pressure, ischemia (occluded vessels), etc. The underlying cause can be treated by dilating vessels,(opening and relaxing vessels; coreg, beta blocker is the appropriate medication).
Years ago my EF was 29 - 13%, an enlarged (dilated) left ventricle, ischemia, etc. and medication reversed remodeling of the LV, reduced bp, slowed heart rate, etc. and today my EF is 59%.