MongoDB Atlas is a newer offering from MongoDB Inc., launched in Summer 2016. Similar to MLab, it offers 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, Asia).
Most notably, the regions missing are AWS China and AWS US Gov regions.
In both (and most others) 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.
Running costs for cloud hosted services can be significantly higher than setting these up in our own servers. What we gain in convenience and time to market, we may have to pay in operational cost.