Erm yes it is. You just haven't found a cable that affects your realtek yet. I have referred my problem to snb forums and the replies there all say the same thing, Realtek doesn't like the cable I am using. My entire house is now Realtek free as I've upgraded those two older PCs and the entire house is now gigagbit. I'm not gonna buy new wires and rewire the whole house again when other non Realtek LANs works fine. Before you ask yes I tested transfer speeds between the non Realtek LANs, both normal network copy/paste and iperf and they show gigabit speeds, up to ~90MBps. So there is nothing wrong with my cables.

My conclusion is, if you only have one problem device/cable, then try a different cable. But in my case I have already wired the entire house, and and every other device works fine (yes even the connection between my router and my unifi modem is gigabit) so I didn't change the cable. I just made sure the next time I replace/upgrade PCs I don't go for Realteks. I have nothing against them. Its just in my case, which admittedly while rare it does happen based on my research, I decided to just move away from Realtek so that I have less headache to deal with.

That's interesting. What brand cables are you using? Strange that a network chipset can be affected by the brand of cables. What affects it? Aluminium, copper, resistance, impedance, conductance?

My 2 main computers are Realteks; Zotac Z77ITX, cheapo USB3 GBe for my MacBook. The MacBook is connected to a router using a Mediatek switch chip and the PC is connected to one using a Artheros switch chip.

This just made me realised that I don't know the brand of cables used in my house. Or I had forgotten. I must have specified something to the contractor. Must go check.