The thing at the end is the Ubuntu "distribution" aka "release" (for that v4.1-rc2, "vivid vervet" aka 15.04). So in this case "unstable" really refers to the ubuntu "distribution channel", not the kernel itself.
But really, you should expect it to take at least a few days to migrate to a more "stable" "distribution channel". On Arch Linux, they often don't migrate it before the "x.x.1" release. That's because they make sure various out-of-tree kernel modules are still working, and they sometimes do need to apply a patch to fix a strange issue related to some hardware that's being used slightly differently, and isn't caught until after the "x.x.0" release which gains much wider usage than the release-candidates.
Ubuntu isn't even a rolling-release distro like Arch is, so what you linked to is a special repo you can add for super up-to-date kernels if you really want them for some reason. But if you have no particular reason, you shouldn't bother. There can always be unexpected complications, usually involving closed-source kernel modules (nvidia, amd catalyst, vmware) or buggy hardware (but of course a linux enthusiast would say that ;)
Regardless of whether you are running Ubuntu, Debian or anything else with dpkg, you can install the latest Linux kernels using http://liquorix.net/ no matter if you are on Old Stable, Experimental or something in-between.
There hasn't been another review cycle, but my expectation is that it will be proposed for merging again this time around. Should be interesting to watch...
Given that kdbus support was merged[1] in mainline systemd, I feel the situation is one of "I see you're not merging my friend's patches... would be a shame if the only remaining implementation of init would support only slow-dbus or kdbus..."
systemd has been using slow-dbus, via libdbus, for who knows how long. Now it will use slow-dbus via sd-bus. Hard to imagine how that could possibly amount to a threat.
Any distribution that doesn't ship modifications to the kernel immediately "adopts" the latest kernel on release, whatever that means. For example, Exherbo had it immediately since users are directed to download their kernel sources from kernel.org; ignoring that, the package for the kernel headers (for building e.g. libc) also were updated nearly immediately, from the main git repo:
commit 629e1b2b1d30608e3b3af74de0c7a220009bba54
Author: Timo Gurr <tgurr@exherbo.org>
Date: Mon Jun 22 18:49:20 2015 +0200
linux-headers: version bump to 4.1
I'm also not sure how you can claim a linux distro is the first without exhaustively checking all others. As you can see, it is highly probable Solus was not the first.
part 1 http://lwn.net/Articles/640297/
part 2 http://lwn.net/Articles/641016/
part 3 http://lwn.net/Articles/642039/