...the more it seems he's one of those writers who get easily bored with their own creations and abandon their novels at about the point where the really hard, more laborious work should begin.

He could use an extra helping of perfectionism, or an editor far less indulgent who pushes him harder. It's not lack of skills or talent, it's lack of work (and quite different from laziness).

His whole notion about his prose being invisible (it's anything but, it's just visibly careless and even occasionally distracting when it gets to the point when for example he makes semantic mistakes with common words or repeats the same metaphor or turn of phrase too many time over the course of a novel without any real intent to create leitmotivs) smacks more of an excuse not to make a few passes of laborious revisions just to polish his prose, not to stop and spend time questioning his story, or deepening his character development. Rather than taking advantage of the facts he doesn't seem to suffer much at all from writing blocks or lack of ideas and that he can draft extremely fast it seems to have become a kind of "it's good enough" syndrome and an excuse not to work on his weaker skills.

But well, if nothing else he might earn one day the Guiness record for the writer who'll have created the most worlds and magic systems in his career...