Hybrid storage combines flash and disk for better performance and lower cost. Most hybrid storage is tiered, meaning that data can "live" on either flash or disk, but there are other approaches that look a lot more like a cache (see Nimble Storage, for example). True hybrid storage would work like Apple Fusion Drive or Hybrid Storage Spaces Direct - A single pool with SSD and HDD where data can reside wherever is best.
L2ARC is only ever a cache and although it can be on SSD, ZFS generally won't use much more than a few tens of GB. ZIL isn't even a cache and is only used for synchronous writes. Check any ZFS tuning guide and the gist will be "just buy more RAM or create an all-SSD pool" rather than trying to wedge an SSD into L2ARC or ZIL.
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.
A write-focussed SLOG device can massively improve your write IOPS though, and is massively cheaper than having an "all-ssd" pool, while being able to approach it's normal use performance. If you have a sustained heavy I/O load, sure - going full SSD will be your best bet, but for most usecases, you only need peak IOPS performance in short bursts.
I'm sorry, I was referring to hybrid storage as a technology category, not the hybrid disk drives like Seagate Momentus XT. You're right that a 2-drive hybrid (Fusion Drive) will mathematically be less reliable than a non-hybrid one and that those "hybrid disks" haven't lived up to the hype.
Pretty much every enterprise storage solution designed today is hybrid (SSD plus HDD) or all-flash and includes lots of advanced availability features.
> "Pretty much every enterprise storage solution designed today is hybrid"
By this do you mean simply that there is both SSD and disk storage around, or do you mean that the storage system transparently chooses where to store particular things without the apps having to care.
If you buy an enterprise storage frame there are a variety of mechanisms where as a storage consumer you just see a LUN, but in the backend individual extents/blocks/volumes are tiered in different performing areas.
These systems will transparently "promote" hot blocks to flash or faster spinning disk, etc and demote cold blocks based in usage patterns and policy, without the app being aware.
There are a lot of different ways to do it, ranging from tiering within the array to a storage virtualization solution that can tier data across different storage platforms. In one case I worked on a project where that virtualization tech was used to consolidate 10 data centers to 1, pretty much transparent to the end users and mostly transparent to the folks running apps. (Exceptions were ostly apps that built their own Storage HA)
L2ARC is only ever a cache and although it can be on SSD, ZFS generally won't use much more than a few tens of GB. ZIL isn't even a cache and is only used for synchronous writes. Check any ZFS tuning guide and the gist will be "just buy more RAM or create an all-SSD pool" rather than trying to wedge an SSD into L2ARC or ZIL.