One of the automatic weight assigning paradigm is to assign vertices by distance to the bones; this works well for some meshes but if there are two far-connected bones close to each other (e.g. arm is right by the legs) it falls apart easily. In practice automatic assigned weights are often a good starting point but almost always needs manual changes.
I've done a decent amount of experience with rigging/kinning and I think artists will keep wanting to do manual skinning to get the deformations they desire, which usually doesn't exactly match an algorithm.
However, automatic methods are rigging are incredible for those who are inexperienced/learning and also times when you just need to test out your skeleton/joints.