Off course these disks do not know anything about the filesystem at all; all they do is look at the raw throughput. There is something to be said about the OS knowing which files are worth caching and which not; but in the end the cache on the SSHD will come to the same conclusion as it simply keeps track about which blocks of data are most often requested. Plus, having an SSHD doesn't magically sabotage your system cache.
Like I said, you may find all sorts of reasons to not like it, in practice the thing delivers on its promise. It's far from perfect and it can be fooled into taking the wrong decisions but that's not a terrible thing to happen as it will 'heal' over time.
Example : my laptop used to have a Momentus XT and although the older model has only 4Gb cache, it booted faster, had Visual Studio open in A LOT less time, could bring Excel up in 2 seconds, same for SSMS, Firefox would load faster, etc ... I very much doubt all of those programs fit in those 4Gb but still it managed to find out which were most 'important' as the loading times of ALL those programs (os) were remarkably lower ALL THE TIME vs my colleagues' machines that were pretty much exact copies safe for the fact they had the original 'ordinary "black" whatever disks". At a certain point in time I had to run a virtual machine for a while and -not entirely unexpected- after a couple of ''virtual reboots' said machine also started up remarkably faster, programs inside it got faster to load too etc... but all of this came at the expense of slower loading of eg. VS when started 'locally'. After the VM-project was finished it took a couple of days for everything to turn back to normal and I guess if I had kept the virtual machine and started it up again then it would have been its original 'slow' again.
Currently I have Samsung EVO Pro + that WD Scorpio Black in the media-bay. The SSD feels 100x faster than did the SSHD but I noticed that having the XT as data-disk (used pretty much only for backups & multimedia) didn't really make much of a difference vs a normal disk as the access patterns were simply not appropriate for it. So I put it in my little file/media-server at home where it makes more sense.