In other words, rather than being skeptical, as I usually would be on something like this - I'm quite optimistic.
I'm optimistic on Mistborn. Everything about that seems very doable.
That will live and die on the casting though, given the tight focus. If they get someone in charge of casting like Nina Gold*, they're well, uh, golden. If on, the other hand, they get the sort of people who have cast a lot of Netflix projects, it'll be a car crash. Apple TV has usually done pretty well on casting (it's complicated because it seems like the don't micromanage casting the way Netflix and Prime apparently do, but rather let the production companies handle it for the most part), so I'm hopeful here. The major worry re: casting is the level of power that Sanderson has. Not that he's irresponsible or w/e, but sometimes people with the final say on casting (which I imagine he has given the Variety article) have, whilst trying to be helpful, lead to some pretty awful casting decisions (this does include some experienced directors - but almost exclusively ones who have a long history of mediocre-to-bad work). We'll see though.
I'm not optimistic on Stormlight, because however helpful he is, it's an extremely hard project to adapt (in part because of choices he himself made in writing it - it's very much a book series which relies on that format, not just "a story" like some novels are), and whether in live-action or animation, would be ludicrously expensive to adapt, and be decade or more of commitment. Even if everything that can go right, goes right, it might never go beyond "Well that's a workmanlike adaptation of a series that's too esoteric, extended and nerdy to really hold the audience that level of financial and time investment would demand". However, I do think Apple TV might realize this, so maybe it's not worth worrying about. It might be something to consider much later on, when Stormlight is either finished or significantly closer to finished. I mean, on a positive note, I will say, Apple TV, unlike Netflix, wouldn't commission it with the full intention of cancelling it after 2-3 seasons.
* = She cast Rome, Game of Thrones, Wolf Hall, The Crown, Top of the Lake, Chernobyl, Bad Sisters, Andor, Slow Horses, Baby Reindeer, some of those just being like, the best-cast shows on TV even I don't like them, like The Crown or Wolf Hall, and some of them being very difficult casting assignments like Chernobyl. Other ones there's some just some really brilliant casting like GoT, Andor, Slow Horses. I notice Slow Horses and Bad Sisters are Apple TV shows.