I can't see Zahard mid diffing any of the FHs considering their hype. He is the uncontested best among them sure, but if he can mid diff one of them, can they really be considered fighters at a similar level? The only Irregular in the Tower who can push him to extreme difficulty should be Urek, and high difficulty for all the others.
But among them, some fights might be trickier because of matchups though overall it's probably not very important. I don't think they can change the outcome in one vs one.
As for countering Disconnection, I don't see how you can really do that. It cuts anything, the simplest way to deal with it aside from dodging is healing and they can probably all do that(Still unsure if you can heal from it) but it's still very annoying to deal with. And even in this case, a good hit to your vitals and you are done.
Other than this way, you'd need an ability to absorb Shinsoo or negate it. But still, durability doesn't matter against it. Even if you are Zahard, Yurin or Bloodmadder, once hit, you are slashed.