The advantage being less load on each pci-e cable, less chance of overheating?
I take it this 2-to-1 cable is strong enough to handle the power requirements, and a Neptune would need 5 of them in total?
What are HCS pins?
The answer to your first question is yes.
The second question is yes..to a degree. Yes in that the cables from your PSU will see less load and therefore less heat/chance of melting. This adapter as is with regular pins and 18AWG wiring still has the same risks as if you used a regular PCIe cable. The difference is that it is cheap and easily replaceable.
The upgraded version I have in mind will probably be overkill for this application but that is how I roll. Molex makes a line of minifit pins suitable for high current systems that allow up to 12A instead of the regular 9A.