The general consensus used to be that questions should be either closed (i.e., all the knowledge is disclosed in the interface) or open (any in-between doesn't help to organinze and conceptionalize thought) and that UI elements should signal this appropriately. Now, with autocomplete, there's a use case for this. But there seem to be two types of users, those who make ready use of this and those who ignore it completely, maybe hinting at the question being still open.

(We may observe how the implementation of datalist reflects this: it's still a simple textbox, i.e. input element, and the list is attached as an add-on, rather than being a list type of its own. It's not a list standing on its own, but an enhancement to the input field. So, rather don't use it as a control element for lists?)

PS: The point was really that, if you were to google "worst interface element", you got loads of articles about the combobox.