Flash Fill/Spill

Has anyone had any success with Flash Fill?
I've had to turn it off because it doesn't seem to be able to handle any serious data. It'll find ridiculous patterns and then proceed to automatically dump the generated rubbish into thousands of rows.
I've also had it where it populated the whole column with the column heading.

Same with Spill. Usually I find this is the result of a typo in the formula, not an intentional outcome?

Seems like these things address problems which didn't exist and force you to interact with a not very well executed AI.