That's only valid for Rand, and arguably Rand's voice in the books is one of those Sanderson managed fairly well on the whole.

The characterization problems with Rand are all fairly minor (but for fans used to analyse the text, they still stood out - and as they piled up, they become fairly distracting when you read). The average reader almost certainly didn't notice many of those). Given a few more runs of re-reads/analysis/annotations, Sanderson would have smoothed them out. He did a re read taking notes, but one isn't enough to cover all aspects, and Sanderson focussed on some aspects. And he relied a lot of his fan general knowledge of the books, which was good and bad (but a necessity on his schedule). An example of these minor distractions is for instance when Sanderson has Rand notice and comment to himself the minute details of everyone's clothing. "Very WOT" many would say, but someone who's paying more attention would notice this is actually out of character for Rand, especially in the later series. Rand mentions his clothes, and Min's. Jordan would never have had Rand pay attention to the minute details of Davram Bashere's clothes. Sanderson did not have time to analyse the POVs that way, and it's no very surprising he added to his writing of Rand stuff that from memory he found typical of Jordan, without going as far as analysing if that was typical of Jordan's Rand in particular. He got the motivations, the relationships (incl. LTT to Rand and vice-versa) mostly right, that's obviously what he took his notes on during his re read.

Egwene wasn't so bad either (but not as good), but that owes much to Jordan who had left quite a bit more material for her for TGS.

Sanderson nailed Perrin down pretty well, the main problem is that he wrote him as the Perrin he knew the most, which is the early to mid series Perrin (he had read the mid to late series once or twice only), so the tone and many reflections were off. Again, a lot of people hate Perrin or find his late series storyline tedious and don't read it closely, so they would't have noticed this. The details/subtleties were of course a bit wrong too. Perrin always took a great deal of notice of his surroundings, especially in the countryside (it's from his POV, and Nynaeve's, we got the most descriptions of trees, landscapes, flowers, fauna etc. - also to mechanical devices. He's another who doesn't pay attention much to clothes, except Faile's)... Perrin changed very suddenly when Faile got kidnapped. Jordan changed his voice. Most of what Perrin still noticed around him, and it was a great deal less than before - and without his usual interest, he found a way to bring back to Faile.

Mat was really quite bad in TGS, remotely better in TOM. It's not surprising, Mat must be a bitch to write, probably one of the most difficult characters for Jordan himself. Even Jordan let Mat "stew" a very long time, describing him from other people's eyes before finally tackling him (he didn't get a POV before book 3, quite a few more in 4, and became a regular POV character by book 5. Like Elayne) and get into his head and his way of thinking. The primary trait Mat inherited from Jordan himself (who was often his antithesis, being more like Perrin in personality) was his humor, and that's terribly difficult to mimick someone's else humor. Among other thing, Sanderson made Mat too conscious of being funny and too intent on being funny. Jordan's Mat is at his funniest when he intended to be serious, or when he was cynical or ironic. In TOM, Mat alternated being "on character" and "off", again because Jordan had left bits and pieces of fairly advanced Mat scenes.

Among the main cast, Sanderson also struggled a bit with Nynaeve, though she was often quite good, then he struggled massively with Aviendha, who is more like a caricature of Aviendha, and with Tuon who was pretty much off character.

It's with the "important" and less important secondary characters Sanderson had the most problems. Tam, Cadsuane, Berelain, Siuan were full of much more flagrant mistakes.

On the whole, though, the biggest flaw of characterization, and it was true for all the characters, is that Sanderson struggled to emulate Jordan's cleverness. Jordan had a superior IQ, and often made his characters's thinking fairly clever (that made their blindspots a bit overdone and fake - and he wasn't at his best faking the thinking of people far less intelligent than he was. He was much more "natural" writing people with superior IQs, the likes of Moiraine, Graendal, Mat, Siuan, Egwene - she lacks experience, not raw brains, Cadsuane, Lelaine and Romanda, Verin and so on). Sanderson is intelligent certainly, but not at Jordan's level. That showed a lot in the thinking of several characters, who visibly have dropped many IQ pointd, and ended up out of character as a result (Cadsuane, Siuan and Graendal are some who suffered the most. All three are fairly bad and unrecognizable written by Sanderson. Demandred and Moridin are very likely to follow this trend. I really don't look forward to Demandred POVs written by Sanderson, and I certainly hope we're done with Graendal as she was completely off in most scenes written by Sanderson).

Sanderson's storytelling has been influenced by Jordan's certainly, but his writing is totally different, and his novels are completely different (it's completely unfair to compare a transitional book like COT to anything Sanderson has written. You'd have to give Sanderson an outline and notes about everything Jordan needed to "place" in COT to see how he would compare with Jordan at this sort of books. If AMOL is anything to go by, instead of the slow paced scenes where for instance Elayne learns of many developpments all around during a bath or a long conversation with disgressions, Sanderson would have added tons of shorter scenes to show many of those events on screen, ending up having to split COT in two books, both also with extra "episodic style" action scenes. He would not have had 4 Faile scenes, for e.g., but 12 shorter POVs of Faile through the book to convey the same exposition and story/character development points Jordan could do in much fewer longer POV. Sanderson is far more fast paced for sure, but he needs more scenes and ends up with more pages to accomplish the same goals . Jordan's AMOL would have been less eventful, with longer scenes and many secondary events referred to only and tons of ellipses (just like he has done in KOD.. a month's worth of Egwene's storyline is told in one chapter! Sanderson would have written one full scene for each event in that chapter).

Sanderson avoids certain type of scenes in his own novels, which happen to be the same sort of scenes he seemed to have the most problems writing for WOT, and that alas happen to be one typical aspect of Jordan's writing (those slow paced scenes in which Jordan accomplished tons of things, sparing him the trouble of showing some events in details. Aviendha might have gotten one or two POVS, maximum. The rest of what Sanderson had her do from her POV on screen, Jordan would only have shown as allusions in other POVs of characters who noticed Aviendha's ongoing behavior).

Jordan could do episodic, faster paced, more action oriented novels (TGH is exactly that) - though certainly he wasn't as good as Sanderson at it (Sanderson would make a great screenwriter, better than he is as a novelist. Jordan would have sucked at that). Jordan didn't write the later books of WOT like this by choice, and he was certainly much, much better than Sanderson at writing books like WH/COT/KOD. Sanderson "cheated" and returned to the tone of the early series, mixing it with elements of the storytelling style of the late series, and the result often felt quite awkward (TGS was almost devoid of secondary character scenes, for instance and they played extremlely minor parts in the main characters's scenes. Some liked that, but no one can say truthfully it felt natural for a late series WOT book).