Hmm...
I think I use YNAB to allow myself nice things while knowing I'm saving. I guess it's because I've got a saver mentality instead of a spender mentality. So I don't think the challenge would be useful to me...
YNAB gives me permission to eat out with coworkers when they invite me on Friday... Before YNAB I would feel bad not to eat a packed lunch.
I'm just not someone who buys unnecessary things on a regular basis. (I lean more towards splurging on outdoor gear once or twice a year.)
I like budgeting because it meant I didn't feel guilty buying two board games on FB marketplace this month even if I knew it meant postponing a purchase of an epilator until a future month.
I've tracked all my spending since I first moved out of my parents place for university... I think I would struggle not to. My anxiety loves loves loves worrying about money. Forget the fact that I could have no income for six months and live my usual life...