Currently we break some guarantees of atomicity by using separate WUOWs for each op and for the final logOp. This can result in a server performing the modification but not adding it to its oplog if the server is shut down in the middle of an applyOps.
We can't use a single WUOW in the general case because applyOps needs to support operations like building an index that are illegal inside of a WUOW. However, it should be possible to detect if all ops are simple single-document CRUD ops and use a single WUOW in that case. This would cover all uses of applyOps that are used when writing to the config servers.