I have been using a pair of Crucial Crucial MX300 275GB 3D NAND SATA M.2 (2280) Internal SSD drives as a cash drive in my Synology DS1517+.
My guess is that due to the constant r/w action these drives have been worn out very quickly. In less then 1 ½ years of use in my home network they degraded to less then 5% and started to fail. I was not expecting for SSD’s to fail this way.
I am not planning to treat M.2 SSD’s as a consumable product and will use the PCI slot for a 10GB Ethernet card and use the dedicated Cat6a line to the workbench for quick back-ups and downloads of large files. Using a 10GBE USB C adapter.