When using the Redwood storage engine the main benefit of the Directory Layer becomes the ability to move/rename directories and having smaller keys in network messages (though some of these may eventually use prefix compression).
The main benefit of not using the Directory Layer is avoiding additional reads to resolve directories to their shortened unique prefixes. It is also worth pointing out that without the Directory Layer it becomes easier to move/copy part of the keyspace from one FDB cluster to another, which can be useful for multi-cluster deployments. With the Directory Layer such a move/copy must involve reconciling the likely-different directory → prefix mappings of the two clusters and possibly translating the copied keys to match the destination cluster.
All releases of FDB 7.0 will have Redwood available for use as a storage engine with the name ssd-redwood-1-experimental. This means you can configure it as your cluster’s main storage engine (but note the experimental label!) or you can use the new TSS feature to test Redwood against your workload with zero risk to durability, availability, or performance of your FDB cluster.
While Redwood is still marked experimental, in practice it is very stable and performs very well. There are no known bugs, and the code has passed many millions of correctness tests. It is still marked experimental mainly because it is not being used in production anywhere yet, and there are some minor but not-backward-compatible changes being made in it for FDB 7.1.