Unfortunately the bolded is very much open for debate. We have two installments in the WoT that offer almost zero rereadibility at this point. Unpolished prose, lowest commen denominator plotwork and poor characterization rear their heads all two often. All of that without getting into the numerous mistakes and continuity errors that Team Jordan have admitted exist. No one expects Brandon to write as good as RJ, we do how ever expect him to write as good as Brandon can and that most certainly hasn't been the case. The verdict is still out in regards to his legacy in relation to the WoT.
As an aside I am interested to hear how you think he has grown as an author? Most say he stalled out after the first two Mistborn books and the initial praise of TGS(based on intitial fan gratification as the sentiment as shifted hugely sense) hurt him in the long run. Perhaps if fans had offered a more reasoned critique from day one we would be in a different situation than we are now. I hope that AMoL is his best work yet as Team Jordan have owned up to mistakes and claimed they need to get this one "right". RJ deserves no less.
First bold. I had no problems reading the the WOT installments he wrote. Only thing that nagged me was Matt's character in the first book. The voice was wrong. I think many here, and I'm not knocking it, read WOT as though they read a history book studying for an exam. I do not do that. Read for pure enjoyment. And although I love complicated stories, I don't try and figure them out as I read them. I let the story come to me as it is paced in the book. So while I can understand that view point, I disagree in the unreadability claim. Nor have I found a majority to agree with that viewpoint as you insinuate.
Second bold. I found the Mistborn series entertaining. Not great works. I found Warbreaker somewhat entertaining, less than Mistborn. While he is great at creating unique magic systems, they don't enhance a story imo.
The Way of Kings is far and above any of his other work including his WOT additions. I think his work on WOT has influenced his prose in TWOK greatly. By having to pick up RJ's torch he has learned a great deal about writing. After reading TWOK and being thoroughly impressed with it I have great confidence that AMOL will be by far his best WOT work.
All that said, it's opinion. I'm just offering mine.