L2ARC and the ZIL can be great in many storage situations, but not all.
We have a TrueNAS appliance with a 480GB L2ARC and a small 120GB ZIL (never fully used) used for image/document storage by our ECM suite. Our metadata usage on the filesystem is astronomical due to having billions of small (<16KB) files, the L2ARC may not do a whole lot for actual data (since most of it isn't hit frequently enough to be eligible to be stored in the ARC/L2ARC) but it's instrumental in maintaining performance with such massive amounts of filesystem metadata.
We have a TrueNAS appliance with a 480GB L2ARC and a small 120GB ZIL (never fully used) used for image/document storage by our ECM suite. Our metadata usage on the filesystem is astronomical due to having billions of small (<16KB) files, the L2ARC may not do a whole lot for actual data (since most of it isn't hit frequently enough to be eligible to be stored in the ARC/L2ARC) but it's instrumental in maintaining performance with such massive amounts of filesystem metadata.