"Is a creator legally obligated to fulfill the promises of their project?
Yes. Kickstarter's Terms of Use require creators to fulfill all rewards of their project or refund any backer whose reward they do not or cannot fulfill. "
Most aren't legally persecuted unless it's a large heap of money and there's evidence of scamming and funds not being used for the product. If the backers take things in their own hands it'd be considered a civil case.
In most cases the creator is just outed as irresponsible and biting off more than they can chew. Those who give the occasional vague post every other century or never post at all because they don't want to admit failure are the ones seen as scummy.
Reputation takes a hit either way.