I would only suggest fast ssds for metadata/special vdevs.
The downside is that if that vdev fails you lose all the data on your pool regardless of redundancy.
This is why I suggest a minimum of a double mirror for metadata/special vdevs.
I've also heard that its not very useful for unless you have 10s of thousands of folders inside tens of thousands of folders.
another drawback is that currently there is no good way to rebalance vdevs if you add the metadata vdev after already having data on your pool. however in the future once truenas 26 or 27 (I forget which) rolls out it should be possible.