MongoDB Atlas is a newer offering from MongoDB, Inc., and was launched in summer 2016. Similar to mLab, it offers the deployment of single-server, replica set, or sharded clusters, through a web interface.
It offers the latest MongoDB version. The only storage option is WiredTiger. There are multiple regions for each provider (US, Europe, and Asia).
Most notably, the regions that are missing are the AWS China and AWS US Gov regions.
In both (and most of the other) providers, we can't have a cross-region replica set. This is prohibitive if we want to deploy a truly global service, serving users from multiple data centers around the globe, with our MongoDB servers being as close to the application servers as possible.
The running costs for cloud-hosted services can be significantly higher than setting them up in our own servers. What we gain in convenience and time to market may have to be paid in operational costs.