Since 2010, I've backed over 150 Kickstarters, of those, 43 have yet to deliver.
However, of those 43, only half-a-dozen so are failures or at worst, unacceptably late. Most of the ones I'm still waiting on are recent Kickstarters than aren't near their completion date yet (and some of those have actually started sending out rewards, like the latest Numenera one and the DCC reprint!).
As I go back through them, I realize some of the ones that are really late, I no longer care about enough to follow-up and see if I'm ever going to see a return on my investment.
Ironically, I am one of the few backers that actually got what I paid for out of a Ken Whitman-affiliated Kickstarter: I backed the Knight's Tale card game and got my rewards within a reasonable amount of time (I also got one of the Rusty the Rust Monster plushies despite the C&D that shut down that Kickstarter some months after it funded).