Haha thanks. 6th edition has really played into my hands never having been a fan of armies consisting of one of each unit type in a codex (ok exaggerated for effect but you know what i mean) or mech. At the moment I worry about CHAOS:
helldrake spam wiping out my troops/being pretty unkillable
Wholetthedogsoutlists (20-60 khorne hounds)
flying circus's
[arguably slaanesh although
tbh they will have even more fragile troops than this list without the resistance of the riptides so i wouldnt say this is anywhere near autolose - might have to play this build soon

could be quite close!]
obviously necrons (wraithspam, annihilaiton barge spam, flier spam or all of the above!)
balanced eldar but just because having not played the new codex they scare me and dark eldar allies are NOT good for T6....
on the other hand although it will struggle
IMO it can survive most of these lists to an extent. And to answer your question about who will kill the night scythes - my other riptides
Your Riptides don't have any resiliance against Slaneesh Rush builds. Too much rending. They are fragile, but a Slaneesh rush list banks on numbers for durability, and you don't have the volume of fire to stop them all.
Riptides are actually pretty good against a Dogpound style list. Only the Heralds are really a threat. If a Khorne Dogpack is threatening your lines, it is sometimes a good idea to charge the Riptide in. Trust me, I run Khorne Dogs (a lot), and the only way I've killed Riptides with them is through multi assault. (which is actually my preferred way to kill a Riptide)