I hedge a bet that you have no real experience of Fusion Drives. FYI they run at the speed of the SSD, NOT the HD. This is because all data is taken from the fast SSD cache with delayed write to the HDD. In my case, as it is running at full SATA 3 speed which is around 500 MB/s. This is the only limitation, the bus speed, not the drive speed. To experience faster SSD speeds you will need to connect them to faster busses, PCIe, M2, etc., which are the latest connection formats available. In my case, 2011 machine, SATA3 is the fastest its got. If you buy a more recent machine with a Fusion Drive I wager that the SSD will be connected by PCIe or M2 so your FD will run at that speed in that case.
The other point you raise regarding failure, all drives are prone to fail be they HDD or SDD, ok SSD have a proposed longer lifespan but that is not a guarantee of non-failure.