ARCore is well suited for demonstrating 3D objects on surfaces that could be used mostly in product sales, real estate. It can be used to measure objects or a room to make a 3D architectural model out of it, although it's not 100% accurate it can give a pretty good idea. It can be used for games and marketing reasons ARCore is less appropriate for human body tracking, occlusion, and large-scale augmentation where there needs to be a large-scale scanning.
Definetely when needing an image target based AR experience, I'd go with Vuforia Engine, the implementation is pretty straightforward (at least when using UNITY SDK), it is well documented and there are plenty of examples to start with. I wouldn't recommend it yet for tracking objects or environments if it is for commercial purposes, because it tends to fail from time to time from my experience.
Vuforia Engine has limited functionalities compared to ARCore. It has image-based detection and augmentation which requires users to have the anchor image with them for the augmentation to properly occur. ARCore solves that and it's one step less to reach the user and really convince them to try it.