And this happens for all vendors/models of SSDs you put into these failing boxes or just the 2500 SSD?
This is really a longshot but there have been revisions of the Intel NUCs during the year regarding SATA compatibility (to improve SATA port signal integrity / to increase drive strength) as described in:
https://qdms.intel.com/dm/d.aspx/0A574169-F4F9-45EB-9211-7A43CF966F5F/PCN113140-00.pdf
https://qdms.intel.com/dm/d.aspx/AC9C0719-A458-4D4C-A9DE-F7F24375D169/PCN113335-00.pdf
From which revision are your boxes and could you perhaps find a different revision to see if the problem exist also at that revision?