A while back, I wrote down several of the mechanisms that I use for keeping track of things that need to be done in a Scrivener project. I think the most important concept to understand—within the narrow scope of tracking work relating to writing toward a goal in a project—is that in my opinion the concept of a separate checklist mechanism is as obsolete as the concept of a spreadsheet used to keep track of details.
Scrivener gives us the tools to record the types of things we would use a spreadsheet for, like keywords, text fields large, small and custom, checkboxes even, dates and list dropdowns (if Label and Status are themselves too limiting). And of course the spreadsheet was a replacement for boxes of index cards—a tradition Scrivener tips its hat to.
Likewise Scrivener gives us ample tools for tracking what we need to do yet, embedded right into the framework of what we are working on itself. The simplest example of this is the provided “Status” example dropdown. Rough draft, First draft, Second draft, Done… we’re already tracking thing in an embedded fashion in a simple way with that. I mentioned how each section in the binder can have a checkbox, at the bottom of that post is a method that takes that idea all the way to a separate checklist—if you really do find that more useful than the so-called “embedded to-do system” I described otherwise.
To speak on that option specifically: to my mind, at some point the question fundamentally becomes: what advantage is there to having your to-do line with a checkbox in a text editor, as opposed to in the outliner? The answer is, not much. Just about the only thing I can think of that a line in a text editor is better at, is that with that you press Enter once to add a new line, and with the other you press Enter twice. 
And for that small price you get all of the metadata we’re talking about to help you organise your to-dos, you get to throw them into collections along with the stuff you’re working on, you get to compile them for making reports, you can create a network of links between them, tag them, sort them, even turn them into a list in a text editor if you really want, and so much more…
Somewhat of a tangent is this how-to on tracking revisions. While the use case may be different, the idea itself is something flexible enough to be applied to to-do tracking. Make a row in the binder for the thing you need to do, and link it to the things in the book that are related to getting it done.
Here is an older discussion for this same request. Normally I would merge them to avoid sprawling duplication of conversation, but in this case the thread is so old it doesn’t really showcase anything Scrivener can currently do. Back then we had to use workarounds to get checkboxes in the outliner.