I'd start saying that it is impossible to really have a balanced character (hint on DST). Unfortunately, the more you try to balance people and at the same time you keep adding more features, the less likely you're to succeed in having a balanced character.
My favorite character got to be Wickerbottom, mainly because I'm insomniac as well. That she has books just makes it all the better. If you say her downside is managing Sanity I'd have to say that is the same for all of us. I hardly sleep regardless of the character, and I'd much rather spend my low sanity farming for Nightmare Fuel than sleeping, and dying is just another part of the game. Spoiled food hardly matters when you can have Dragon Pies, and most of the times that's mostly what I eat.
WX-78 would easily be my favorite did I not identify myself with Wickerbottom so much. On the long run it makes the OPest character, simply because it can upgrade. Not to mention the spoiled food eating, I'd say that wetness is more of a bother than a real penalty. Dealing with lighting hardly matters, considering that during winter you get to make an eyebrella.
The thing about Willow, as you say, is due to her "spontaneous combustion". At first I loved Willow, but soon I started to be more annoyed at how often I'd find myself burning down my forests and how little I needed charcoal compared to boards and logs. Her lighter hardly matters in the long run, being next to useless when you need to see in the dark, being useful only in the beginning when you need to find a good location for a base and in the odd occasions when for some reason or another you forgot to keep some grass and twigs with you in case of an emergency. Also, sanity thanks for fire? That sounds terrific, if you actually had the luxury to spend a good time next to something aflame, and sleeping did not recover more sanity.
Talking about SW, the new characters seem pretty bland to me. Though Woodlegs is nice, I hardly see the other characters as useful. Wilbur... is... what? A monkey, seriously? And which main perk is running? In a world where finding good stretch of land is close to impossible? The best place where you'd make good use of Wilbur is if you decided to make base in the Volcano. Then you might as well make use of your running. Other than that, monkeys will hardly matter.
Warly cooking pot, and consequently his food nitpickness just means I'm eating Meat Stew whenever I need hunger points. For healing, nothing like Honey Poultice, specially when there's a huge Tidal Marsh island for the picking. Sanity could have been a bother though, were it not for some of the ridiculous huge sanity food which are not much of a problem at all to make added by SW (Gumbo, Surf and Banana Pop). All in all, Warly is more of a bother than really interesting to play.
And I could go on and on talking about all the characters but I believe this is long enough.