I definitely think Brandon Sanderson had a LOT to learn about writing characters, and that was definitely not one of his strengths in his early works. I haven't read Elantris, but looking at the first Mistborn trilogy and Warbreaker, the worlds and the magic are far more interesting than the people. I think that's definitely a fair criticism.

However, I also think he was aware of that and made efforts to improve. It's still not one of the things he's best at, but the Stormlight Archive, the second Mistborn series, and even the Reckoners books show a marked improvement in characterization over Sanderson's earlier works.

As for his contributions to the Wheel of Time, none of the things people commonly complain about stood out to me at the time I read them (though admittedly it has been awhile).

Spoiler: nothing really specific, but spoilers on the general direction of the series
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What I did like about Sanderson's three WoT books is that he got things moving again. It really seemed to me like Jordan had lost control and just kept letting the scope of the story get bigger and bigger. There were so many POV characters that even though these books were closing in on a thousand pages, individual characters' stories often felt like they barely progressed at all.

Brandon Sanderson had to take Jordan's notes for A Memory of Light and make them into three books to finish out the series. Had Jordan lived, I'm not sure he could have done it in five.