As someone who was stuck in K12(GCA) for several years, I'll give you my two cents on the system and why you should avoid it like the plague(this doesn't apply to other online systems although some things will most likely overlap).
First and foremost the best way to describe the educational/teaching 'methods' is that it's completely impersonal. You're almost completely cut off from teachers outside of optional online class sessions known as Elluminates. Trying to get any real degree of connection going with the teachers is frustrating and feels pointless a lot of the time.
Secondly the only way I can really describe the curriculum and the way lessons and quizzes are set up is utterly broken. For lots of classes the lessons don't contain everything that will be included on the tests/quizzes that are scattered throughout each block(Unit) of the Semester. The quizzes themselves are probably one of the most frustrating things about the system as a whole, the tests feel sloppy and many of the questions feel vague so it's easy to feel like you could have easily scored better on a test if the questions hadn't been worded so poorly. Finally the most dangerous part about it is how little the teachers actually care, with no firm deadlines for tests,papers, or projects outside of the end of each semester it's very easy for younger kids/young teens to fall into the habit of not wanting to do any of the work at all and then gliding through it all at once in a couple of weeks towards the end of the semester. For the most part kids don't have the proper restraint to avoid working their way into this groove, because of this you have to constantly be working with your kid to keep them on track. That won't be easy if you're single and working day shifts.
In short K-12 has amplified all the negative aspects of public schooling with few to none of the benefits you get from homeschooling. Avoid it if at all possible.
That's just my two cents though, feel free to prod me with questions if you would like more in-depth details about parts of the system.