SimpleDB is deprecated, more expensive than DDB, and kind of weird to use. Backing your keystore with a deprecated service just sounds like a road to many sleepless nights ;)

The utility does depend on three external services: DynamoDB, KMS, and IAM (for permissioning). This might sound like a lot, but the target usecase for something like this is a fleet running in the cloud. If you're already running in EC2, then using IAM roles to distribute AWS creds is a no-brainer and very simple. KMS and DDB are also not arduous things to depend on if you're already in the cloud.

For credstash, you need to set up your KMS key (and set whatever policies around it make sense for you). Once you do that, there's nothing to manage. Sure, you depend on DDB, but credstash creates the table for you, and unless you need to dial up more throughput (which is a mouse-click operation), you never have to touch it.