CAlling spiritual companions, demons and the such is hard but not impossible. It requires not only tearing a rift to the maelstrom but practically making a portal to another world for the entity to enter. It's hard but dismissing the entity is easier. 

Conjuring ethereal items would similar to enchanting as you are trapping magic within the world. It would have a time limit BUT that time limit should suffice if you aren't expecting it to last more than a couple of hours at most. Reason why it's impractical is because of the constant risk of the Wraiths. It´s similar to training a Wyvern. Most consider the reward not worth the effort. 

Making something out of literally nothing is practically impossible due to the magic that it involves. This is why Enchanting something that will last is done in groups, not by a single person.