I ordered 3 cables. Two of them worked fine, but the third not so much. The barrel connector doesn't make a good connection with the NES and Genesis. Both systems work fine with their original power supplies, so it's the cable that's bad.
I contacted support but got filtered into junk mail due to how I formed the email. Eventually I got a hold of someone and they sent me a replacement cable which worked perfectly.
Thanks for the great support!