the chip in general should work, the driver is ahci and part of synologys kernel, is you suspect a driver/kernel issue you could try migrating to ds3615, that uses a older kernel compared with 918+ (you did not write which of the 3 possible types you installed)
might be quality issues with the card or heat problems
the later can be checked by inspecting the heat sink, is it a good fit, as the heat sink is removable you could remove it and check the thermal solution, maybe its a rubber pad and its not working properly, you could try removing it and replace it with normal thermal grease as used for cpu's
if you can send it back and get you money back that might be a solution too, i'd suggest a JMB585 based 5port controller, this chip has pcie 3.0 support,also uses 2 pcie lanes, resulting in double bandwidth compared to the controller you have (max 1000MB/s vs max 2000MB/s)