Hybrid HDDs on ReadyNAS Ultra 2

There aren't any on the compatibility list.

What do you expect to gain? The limiting factor for data transfer is the gigabit Ethernet,

SSHD drives are mechanical drives with a large (8 GB) cache, so TRIM isn't relevant. Whether that cache helps performance much depends on the workload (random I/O on a small working set probably would be faster, since performance would be disk-bound, not network bound).

I haven't seen any test results of SSHDs in RAID-5 or RAID-6 though.

Also, the mechanical drive mechanism of the ST2000DX001 is believed similar to the DM drives, which don't seem to fare that well in NAS RAID arrays. So personally I wouldn't use them.

I am not relay looking for performance gain, but more for reliability, since SSHD use less the mechanical (headers). The most used files are hold on the cash side (SSD).
I have also used ST2000DM001 in RAID for a few years without any problem.
Since the price is almost the same (HD vs SSHD) I would like to use SSHD. Actually the question is: can SSHD be worse than HD on ReadyNAS Ultra 2 ?

Thanks

I don't think they will perform worse than the ST2000DM001s. Not sure about reliability (or whether your thinking about less wear will turn out to be true in practice).

In a larger NAS there could be issues due to vibration,but I don't see that as a problem on the Ultra-1.

These are just guesses though, since no one using these disks replied.