A Redis only system is very interesting but are you doing any kind of replication or clustering and would you recommend it as a main datastore for mission critical user data (accounts, profile, purchased...) ?
You can do it with shared storage and/or replication without worrying about data loss on a failure.
The key to scaling and maintaining HA with redis is using clustering, either built into the app or through something like nutcracker (https://github.com/twitter/twemproxy) and making sure you properly balance.
The performance of redis makes it very worthwhile to deal with the persistence/HA issues.