How old were they? I found that around late-middle school/high school, homeschooled kids really start to fall behind. Maybe it was the limit of parental knowledge, I'm not sure.
In the lower grades, the biggest issue is the parent/child dynamics. When things are going badly, do they spin each other up or can the parent keep things calm. If you spin each other up, homeschool is not for you.
In the higher grades, parents need to know their limitations and bring in experts when needed.
Personally, I adored homeschooling my children.
Honestly the foundational work is just as vital to the parents as it is to the kids as we will be simply guiding them through a high school curriculum that is much more advanced that anything we ever experienced. This is the program we are using, but we have two more years before we begin.
However, having seen kids give up on the independent study route and beg to go to a standard school, I think you are underestimating how daunting it is to start learning high school subjects as independent studies. Fundamentally, arguing with your peers in history class or being able to ask questions in chemistry is really valuable. A video is interesting, but once you get to the point where there is a part you don't understand, it's really hard to make progress.
A lot of the "challenge" level courses seem to talk about class presentations? Is this a remote school more than homeschool?
You are completely correct about how hard independent study can be, in the group our younger kids go to, a few of the older siblings have opted to go back to a classroom settings, not all of them, but every kid is different and with a strong academic basis they can thrive in whatever setting you put them in. More importantly if the students choose to go to college that will almost definitely be a traditional college experience so at some point they will need to be acclimated to traditional school. I hope my children can thrive in the independent program, but if not they will have the skills needed to learn in any environment.
I found that a solid elementary school foundation from my parents, combined with weekly classes and individual tutoring during highschool, was excellent preparation for my university studies.