I put my message here too, new Vuforia user.
Vuforia is working perfectly with the S8 on tracked images, not the problem here.
I've tested several things with ARCore and Galaxy S8, and if I learn something is with ARCore and ARCore unity plugin, once you've put an object on the floor, the tracking is SOLID. Even on a table actually, or every horizontal plane. The object doesn't move a bit, and if you lose tracking by walking away, it returns very quickly once you're back. It's very simple, you track planes, you get planes, you do a raycast, you get a tracked hit, you anchor your object on this hit, it's done. And the tracking is perfect.
I've tested Vuforia Ground Plane on the same device, with same asset, and I can't let a simple asset at the same place and rotation for more than 2 seconds, and I don't even talk about table that are untrackables planes, I can't put any object if I walk a bit. It's not even on the floor actually, it's often too high or too low.
So maybe I was mistaken about the Ground Planes concept, but for me it was simply a Vuforia implementation of the ARCore. But with Unity 2017.3p2 and the lastest version of Vuforia, I'm pretty sure that there's no use of ARCore here, or something is very wrong about the using, because the tracking is really disappointing compared to ARCore Unity Plugin.
What gives me a hint is that the documentation said that supported devices goes to Galaxy S8 but also S7 and S6, which are not supported by ARCore Preview 2 (only Pixel, Pixel 2 and S8 so far). So I guess none of these devices actually use ARCore, or at least, it doesn't use the full capacity of it.
Is the Ground Planes supposed to be an ARCore thing with a Vuforia overlay ? Or it's just a Vuforia thing "enhanced" by ARCore ? Will Vuforia "really" use ARCore in the future on Android ?
Thank you !