I don’t know, it’s going to depend a lot on usage pattern and cache hit ratio. It will probably do a lot more writes than normal to the cache drive as it evicts older stuff and replaces it. Everything has tradeoffs in the end.
Another big tradeoff depending on the cache mode (ie. writeback mode) if the SSD dies, you can lose a fair bit of data. Not as catastrophic as a RAID0 would but pretty bad. And you probably want writeback for the fast writes.
Thus I had 2 SSDs and 2 HDDs in RAID1, with the SSDs caching the HDDs. But it turns out my SSDs are kinda crap (they’re about as fast as the HDDs for sequential read/writes) and I didn’t see as much benefit as I hoped so now they’re independent ZFS pools.