The Sweet Treats Challenge is very flawed.

Well, I think it depends a bit of your luck, also using a good strategy, spending some simoleon and a little simcash obviously helps.

I try to not spend sugar in the first 2-4 stages(depends if it’s a 7,9 or 10 stage challenge). Also always use simoleon if there’s an option to increase % with simoleon.. and I use simcash only when I fail in the first or second stage(thats 5 or 10 simcash).

I’m already at 56/60, if i’m lucky with 2 more challenges I will have enough tokens to finish, then I’m going to use the 10 remaining challenges to get heirloom tokens and fashion gems.

But well, I agree that probably being unlucky and not having simoleon/simcash to spend, it can be a bit difficult to finish.

I am very annoyed with the STS this time.  This weekend I have done 9 challenges so far with a total of 80 rounds.  I always spend sugar/simoleons to get the chance above 92%.  In a normal weekend using that percentage, I loose about 1 per day.  I used to target 90% but that was averaging one lost round per challenge while 92% averages me one lost round per day.  
I have an entertainment budget of $25 per month.  I typically use some of it on The Sims Mobile.  I don't mind spending a little money if the game seems fair and fun.  I really like the game because it seemed fun back when it first launched and I was playing only for free.  Over those first few months, the game worked hard to earn my willingness to spend and commit some of my entertainment dollars to it.  This weekend and this STS has done serious damage to my opinion of the game.
This weekend of the 80 rounds I played, I spent resources to get all percentages above 92%.  I still failed 41 times.  I don't know if they changed the RNG or if they did other tweaking but I am quite cross.  The odds of failing 41 times of 80 when each round has a 92% or higher success rate are astronomically low.  Perhaps they don't value my play or my money.  If this continues, I will simply stop playing.  The hundreds of dollars I've spend on this game over the years of play are a sunk cost.  They only get new money as long as the game is still fun.  Having such a draconic loss percentage is not fun.  If the next STS is the same, it will be my last.  

The game is shutting down in few month and you cannot earn anymore simcash.  They said use the resource that is left which is not enought.  First three attempt already lose at 95% chance to win in sweet treat showdown.  How can not it be fun and a lasting challenge if you lose so many times in a row.