It is well suited for small objects in home/office or familiar surroundings to anchor Augmented Reality Objects. It works well for most e-commerce use cases, travel use cases, games, etc. It is not well suited where depth occlusion is the primary use case or human body anchoring is required or even for large-scale AR use cases.
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.