This reminds me of the hybrid drives. When the NVM failed its was a nightmare to deal with. IMHO it's a bad idea from a stability perspective to be caching off drive to Non-volatile memory.
Your last sentence does not follow from the preceding one. Hybrid drives were doomed by having truly tiny caches, making them not particularly fast (you need a lot of flash chips in parallel to get high throughput), prone to cache thrashing, and easy to wear out the NAND flash. These days, even if you try, it's hard to build a caching system that bad. There just aren't SSDs small and slow enough to have such a crippling effect. Even using a single consumer SSD as a cache for a full shelf of hard drives wouldn't be as woefully unbalanced as the SSHDs that tried to get by with only 8GB of NAND.
