The "Basic Deployment", as described in the documentation, uses a 2+1 member replica set as the application database. As all writes to this database seem to use w=2, loss of one of the data bearing nodes means that Ops Manager is no longer operational. Users cannot log in, and no monitoring data can be ingested.

Our documentation does explain that (https://docs.opsmanager.mongodb.com/v1.6/core/deployments/#basic-deployment), but the hint needs to be a lot more prominent.