I don't have solid numbers, just some experience using this. Ephemeral drives outright fail more often than EBS volumes, however, EBS volumes suffer performance degradation significantly more often than ephemeral drives. EBS volume performance is HIGHLY variable, at all times of day, no matter what load you throw at it. Ephemeral drives are very consistent most of the time.
Both types of drives CAN and DO fail, so RAID-10, fail over, and replication are a must have.
Both types of drives CAN and DO fail, so RAID-10, fail over, and replication are a must have.