[Xposed General] GermainZ:
I'm going to use a simple comparison I understand, since I'm not familiar at all with Xposed's internals (I have a basic understanding, but it's on a very low level I know little about.) :)

You want to modify a human and install a pacemaker (= Xposed; modifies the system (body)).

In order to install a pacemaker, you need to know 1- how the heart works and 2- where you can "plug" the pacemaker (in the correct vein).

You take a human from 2014. This is Dalvik. You've already studied all about him, so you know where the heart is, and you know where each vein is. You know what the pacemaker must do exactly (send electrical pulses). Everything works.

Now a human from the future comes in, called Arnold. But Arnold isn't a normal human, his system works differently. On the surface, he looks the same: his nose is where it's supposed to be, his mouth is normal, etc (= for apps, Dalvik and ART don't really matter; if the app uses documented, public APIs (e.g. "get eye color"), the eye is in the correct place and you'll get a color you know).

Now, if you want to install a pacemaker, it's different. On the inside, Arnold has been heavily modified and the veins are in different places. You must identify which veins you want again, test it, etc. Arnold's heart also works slightly differently, so you'll also want to adjust the pacemaker's pulses. Now the pacemaker works for Arnold, but it's not fully tested/working (rovo said he's got a private build of Xposed that works for KitKat's ART, but not completely).

Another "human" comes from the future. This is Arnold v2. He's based on Arnold (this is the ART seen in the first preview, based on KK's ART), but heavily modified (just like ART). They changed the location of the heart completely! We must find the appropriate veins again, and then connect our pacemaker there.

Arnold v3 comes now. Again, he's been modified (this is the ART seen in the second preview). There aren't *many* changes from Arnold v2, you just need to connect the pacemaker to a different vein.

You discover some images of Arnold v4, and manage to get a hologram! This seems to be the final Arnold version (= final ART version; the sources are not released yet). Looking at it, you can see it still functions like the original human externally (so calling "get eye color" will still work as expected). Internally, though, you're guessing there will be more changes… so you wait till Arnold v4 finally comes to Earth, capture it, then open it up and start checking which veins you want.

Basically, the things Xposed "hooks" change. Is it possible to modify it to make it work for each and every version of ART? Yes, but you'll need to treat each as a different product, in a way… in other words: a lot of time is necessary to modify Xposed and test it for each ART version, because it's still heavily changing. Once L is finally out (not previews), it won't change as much so you'll be able to focus on it once and for all.

To summarize, there are two keys points:

ART has been new/a preview until now, and was still heavily changing. This will almost certainly change once the final version of L is out, and ART changes will become less major.

The final ART version isn't out yet.