I can confirm the cache works fine when the boot system drive is an SSD.
It was specified somewhere the SSDs are automatically excluded from caching, anyway I always excplictly exclude them via command line.
I have two PC both with SSD system drive:
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Crucial 128GB + 3x2TB SATA
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Sandisk Extreme 480GB + 1x2TB SATA + ARECA RAID5 Array 4X750GB
Caching is working fine on both, also on ARECA RAID array (at least last time I checked).