The MBPs and Minis from that year had dodgy connectors for the optical bay. I wouldn't offer that it's unreliable, but, rather pick a SATA II-only drive to go in that bay - if it's an "automatic selection" drive (SATA I/II/III) for some reason the controller will sometimes be SATA III and most-to-all of the time SATA I (read: slooooow, pun intended…).
I've got several Mini Servers from that year and have not have issues.
I went through 4 2012 Minis and each had that issue - the optical bay drive defaulted to SATA 1, and they went back to Apple. I've read on the interwebs and here on MR Forums about the slow data speeds and the recommendation to stick with a SATA II drive (spinner or SSD, if you can find one…) in that drive bay. Because of this experience and what I read I passed on the choice of making a fusion drive out of the Minis and it was easier to send them back…
What I recommend is put the fastest OS-specific SSD in the main drive bay and leave the optical alone. I can only recommend the Samsung 850 Pro for a main drive, having tested a bunch of them for use in my SMB. I love the EVO, but the Pro pretty much blows the EVO away as an OS drive and as a data drive. My favorite bar chart is the one put out by CNet for the 850 EVO:
https://www.cnet.com/products/samsung-ssd-850-evo/2/ - and I've got an 850 Pro in my personal 2012 Mini Server with the stock (slave) 1TB drive in its upper bay after trying an EVO.
My work Mini Servers have dual 850 Pro SSDs installed, in RAID 0, but they're used as video ingest workstations and file server front ends, installed in Sonnet xMac units. I wouldn't call them stock, but the 850 Pro makes those Minis - with basically the same internals as your laptop - pretty much solid, fast computers. Cheers!