As stated, "fusion" drives have only been out for a couple of weeks now, but a few items of note (at least how I see it):
1. SSD's aren't any better or worse than Hard drives when it comes to failures. Flash memory will degrade over time, but their biggest failure point is actually their controller chips failing. Hard drives are mechanical and thus are prone to failure due to motors dying and/or locking up.
2. Technically, just like any RAID 0 setup or Drive Spanning (it's really more of a hybrid Drive Spanning), it would be more prone to failure than an SSD or hard drive by themselves only because now you have two drives that are each prone to failure of X rate and you have combined them into 1 "drive" thus increasing the failure rate. As independent disks, you only lose what's on one drive.
With all that said, in the end you need a good backup solution regardless if you go Fusion or independent disks, you need a good back up solution (i.e. Time Machine) in case of a failure.