It's viable. Drivers exist for most of the interesting storage engines you'd use for user records, leaderboards and the like. You'll need to pay attention to any server-side operations that require heavy CPU and ensure those tasks don't block time-critical operations.

You may or may not get the same level of concurrency as someone's custom-tuned C++ backend, but you won't have to pay an arm and a leg to license it, either. :)