The success rate of CPR is quite complicated. Nonetheless we have some interesting facts.

1. Overall survival from OHCA has been stable for almost 30 years
2. The aggregate survival rate, is between 6.7% and 8.4%
3. 5% of patients resuscitated with conventional CPR and more than 20% of those resuscitated with extracorporeal CPR were diagnosed with brain death
4. OHCA victims who receive CPR from a bystander or an EMS provider are much more likely to survive than those who do not
5. The most powerful criterion associated with survival from OHCA is ROSC in the field. Failure to restore a pulse on scene indicates that the patient will not likely survive to hospital discharge, irrespective of the subsequent sophistication of in-hospital care

Source

1. Sasson, C., Rogers, M. A. M., Dahl, J., & Kellermann, A. L. (2010). Predictors of survival from out-of-hospital cardiac arrest: a systematic review and meta-analysis. Circulation. Cardiovascular Quality and Outcomes, 3(1), 63–81. https://doi.org/10.1161/CIRCOUTCOMES.109.889576
2. Sandroni, C., Cronberg, T., & Sekhon, M. (2021). Brain injury after cardiac arrest: pathophysiology, treatment, and prognosis. Intensive Care Medicine, 47(12), 1393–1414. https://doi.org/10.1007/s00134-021-06548-2