I would not install an SSHD in a RAID. Personally, I wouldn't use an SSHD as a matter of principle, because although you have the advantages of both technologies up to a certain point, you also have all the disadvantages and risks of HDDs and SSDs.
I have not had any bad experiences with WesternDigital HDDs so far (12+ in private PCs and at least as many in the shop-->only one defect).

I would rather advise you to use this one... these disks are designed for continuous use in the NAS...

WD Red (4000GB, 3.5", NAS)

I have 5 of them in my Synology 1511+. You only notice something when you read data several times, for example when you start virtual machines from the NAS (ESX/XEN etc). Otherwise you will not notice any differences to normal HDs. This is because the disks manage the cache autonomously, the NAS simply sees them as a normal HDD. If you want read/write cache in the NAS, it is better to build a smaller SSD into the NAS and then configure it as a flash cache.

I have a Seagate Desktop SSHD with 2000GB installed in my desktop as a single drive. I am satisfied with the throughput and quality.
I can't say whether the HD is suitable for a NAS with RAID-5.