Question Limits of face tracking in the y axis rotation iOS, Android + 3D skeletal tracking question

I've been experimenting with the effectiveness of face/head tracking with AR Foundation...

It appears to be a dependency on ARKit and ARCore for what this range of effective of motion is...and they both appear to rely on facial features (eyes, nose) as far as I can tell..when these features become occluded, tracking no worky.

So far, in eulerAngles, it appears to me, if the nose is 0° boresight, the head cannot be tracked in either direction > |60°| (ARKit) or |50°| (ARCore).

I'll be thrilled if you tell me I'm wrong here. I'd love to be doing this wrong. Is there any documentation which indicates what the thresholds of face tracking are for either of these platforms?

To achieve my goal of tracking the head's rotation in relation to the camera, I'm planning to try the 3D skeletal tracking for iOS. Ideally, I'd like to return the rotation in relation to the camera, so that I can know what the camera angle in relation to the head rotation is. How might I access this rotational data of the head? It would also be nice to know the distance from the camera. I'm not super familiar with the way the skeletons are rigged - would be much obliged if you can share how you might approach!