Ns and Ss are on different "wavelengths", and it can be difficult to communicate with each other, because their communication styles are different. It almost feels like the two were raised from other-worldly cultures. So when a S is communicating with a S, or an N is communicating with an N, it will feel like they're both from the same "culture".
When a S is paired with a S, they talk to each other in straightfoward, literal terms. This is very understandable to both.
Ns give off vibes that are more vague, whether it's verbal or not, and because other Ns naturally communicate in this style as well, they're just on the same plane.
To Ss, it can seem like the N is "holding back" on a ton of words (or details) that should "just spill out".
When I learned about the S/N, it made so much sense... the family members I live with are Ss, and my whole life every one of my friends were Ss, except for one N, and she was the only one where I felt we were so "natural" with each other. Even before I knew about the S/N communication, let alone MBTI, I never felt connection with the S friends and family at all. It always felt like they just weren't on the same page as I was. Actually... I always just felt so far away.
But more back on topic - I guess what Magic Poriferan meant was that a relationship can be difficult (or distant) enough when you don't share the S/N trait, let alone every other letter being the opposite.
I've actually heard that an INFJ/ESTP relationship can be very electrifying (in a good way) though. Not saying this is true; just what I've heard. But for a side note, my best romantic connection was with an ESTP.