After some time, people started to post about problems with the new WD Red drives. People had troubles restoring failed drives, I had a problem where I think the drives never stopped rewriting sectors (you could hear the hard drives clicking 24/7 even when everything was idle)
Then someone figured out that WD had secretely started selling SMR drives instead of CMR drives. The "updated" models with more cache were much cheaper disks, and they only added more cache to try and cover up the fact that the new disks suffored from catastrophic slowdowns during certain workloads (like rebuilding a NAS volume).
This was a huge controversy. But apparently selling SMR drives is profitable, so WD claims the problem is just that NAS software needs to be made compatible with SMR drives, and all is well. They are still selling SMR drives in their WD Red line.
Edit: Here's a link to one of the many forum threads where people discovered the switch: https://community.synology.com/enu/forum/1/post/127228