I am not certain if i understood correctly but it seems it won't give any practical benefits for my needs because I currently have a Mac Mini, SSD has OS & software and most often used data, hard drive has rarely used other data?
Well it goes in that direction.
You would put an OS as well as applications on an SSD while putting media files on an HDD. You are doing so since you assume that it is beneficial if the data you put on the SSD it's accessed faster. This matters much more for applications than for media files, as long as they are just streamed into the RAM while being played back. And many people notice that 128 GB isn't enough since their OS and application data exceed this amount.
Options to optimize this situation are limited. We could start putting applications that we rarely use on the HDD, but unless we have some kind of usage tracker running, this would just be a rough best guess. And we don't even think about spreading application data across several drives, let alone OS data. Although it's obvious that we could free up a lot of SSD capacity if we would only keep the often used data on it. Most applications simply won't cope with this since they work with relative file locations on the file system. But applications don't care about drives. All they care about are file paths. So if the file system isn't a subset of a drive, applications work just as well.
Now this is exactly what Fusion Drive does. It spreads a "partition" (the term obviously isn't correct) across 2 drives, tracks the usage of data and it accordingly on these drives. One might think that finetuning on a sub application level doesn't make much of a difference. But from my experience, the difference between actually loaded application data and total data is huge. Except for some rare cases where some data is analyzed during startup, it's a safe estimate that programs don't need more data being read during startup than what they use in memory. Usually, it's quite the opposite, programs in memory contain a lot of data that was compressed when it was read.
I'm quite confident that most people don't require more than 128 GB of quickly accessible data if the distribution works perfectly. Since no one outside of Apple seems to know how FD works exactly, it's not quite clear how near to perfection the current implementation already works. All I can say is that it works surprisingly well for me. The only time I can even notice the disadvantage of an HDD is when I do stuff like copying a lot of videos or photos from or to my external SSD. When copying over network, USB stick, external HDD, SD card, etc. the FD HDD is never the part that is slowing down the process.
I totally accept that there are people who aren't the ideal target audience of the FD, namely people who have to deal a lot with huge files or those who are very noise sensitive. But most of the critique about the FD is coming from people who seem to address it from a theoretical point of view without appreciating how it actually works.
They often say things like "I got more application data than the FD has SSD capacity", "Anything other than putting the whole OS on the SSD is just stupid", "I work a lot with big files" (it depends much more whether you need quick access speeds for these files), "SSD are much more reliable than HDD" (actual data on that isn't that conclusive, but doesn't actually show a huge advantage for either drive type), and so on.
