That is a pretty broad question. We would need to know more about the applicatoin or the environment. A good RAC installation is quite resilient but you have to remember that there are still single points of failure. ASM is a big one that has caught me out also CRS corruption is another. You lose either of these and there can be problems. I think choosing a HA is very important and you should look at your requirements. Now you didn’t actually mention a database so you could be talking about Postgres clusters or something else altogether. So once again I say please give us more info.