Welcome from a fellow newbie on a budget with schedule constraints!
I don't know which online ground school you saw ads for, so can't comment on the "legit-ness" of that particular one, but I did my ground school all online and for free. It is a lot of work and requires lots of self-motivation, but it is legit and doable. I went looking for ground school syllabi online to see what and how was taught, and then semi-structured my studying around that.
I used the FAA books (Airplane Flying Handbook and the Pilot's Handbook of Aeronautical Knowledge) for my textbooks and fly8ma.com for some videos and other help. And then used YouTube for extra help with concepts if needed. If you search Fly8MA flight training on Youtube, it will come up (Flying 101). You can watch through the entire course on YouTube or you can go to his website. He has a free online ground school using the YouTube videos - basically the videos plus some notes or important concepts listed down below with a quiz for each topic.
Reading through all that (which is pretty dry and difficult to keep going sometimes...but worth it) and going through the course covers pretty much everything you need to know. I would also read through the actual text of the important FARs (91 and 61 and a few others). Then do online practice tests. See what questions you got wrong and figure out why. It will help you see if there are any areas you are lacking in.
Now, the success story.
I went into my first lesson and my CFI tried to give me a ground lesson. It basically was an oral test. He told me he wasn't going to give me ground anymore as long as I came to my lessons prepared for the day. It has worked pretty well so far! And I took my written test last week and scored a 95% so it seems to have worked out well for that, too.
If you got through all that, you should be able to read those FAA books. Should I have just said yes, online ground school can be legit?!