• RAID does not equate to 100% uptime. Nothing can. RAID is another tool in the toolbox meant to help minimize downtime and availability issues. There is still a risk of a RAID card failure, though that is significantly lower than a mechanical HDD drive failure.
• RAID does not replace backups. Nothing can replace a well-planned and frequently tested backup implementation!
• RAID will not protect you against data corruption, human error, or security issues. While it can protect you against a drive failure, there are innumerable reasons for keeping backups. So do not take RAID as a replacement for backups. If you don’t have backups in place, you’re not ready to consider RAID as an option.
• RAID does not necessarily allow you to dynamically increase the size of the array. If you need more disk space, you cannot simply add another drive to the array. You are likely going to have to start from scratch, rebuilding/reformatting the array. 
• RAID isn’t always the best option for virtualization and high-availability failover.