Given how difficult it can be for a trained teacher to successfully manage lesson plans and teach one or two subjects, it's not hard to see why homeschooling is a disaster for a lot of kids who wind up being taught by a single parent with minimal to no education credentials.

Best-case scenario I've seen is that the students wind up pretty un-rounded, and wind up excelling in one or two subjects, while neglecting basically anything else. It makes sense from the perspective of the parent as well. That might be the subject you're most familiar with, and therefore can, even unintentionally, give it the most attention. Another aspect is wanting your kid to succeed, and if you see them excelling in a subject and doing poorly in another, there's a tendency to just keep focusing on that since they're doing so well on it, rather than "wasting time" on other subjects they're not excelling in.

Only 2 people I've ever known have managed to be what I'd consider "successful" after homeschooling. Both went to the same engineering school that I did, and did extremely well overall. However, they suffered many of the same issues in terms of over-specialization, which becomes really apparent when you talk with them for an extended period of time. Usually they also tend to be incredibly socially inept, but fortunately for these guys their parents gave them a ton of opportunities for socialization with clubs and social hobbies, so that wasn't as apparent, but still something that a lot of homeschool students miss out on.

Realistically, I think the only way you could feasibly manage a truly quality homeschool education is with both parents having an in-depth education of 2-3 subjects (like 5-6 years minimum), in addition to at least some formal educational training. Then, bringing in home tutors for the subjects they likely won't be able to cover nearly as well to supplement them. Finally, having them enrolled in an absurd amount of sports/clubs/hobbies to make up for the 6 hours of social contact with other kids they're missing out on.

With that being said, it's a fuckton more effort and money than almost anyone is going to actually put into a homeschool education, and 98% of the time it's going to be a stay at home parent with minimal to no credentials buying lesson plans off the internet, brushing up on them the night before, and likely giving lackluster instruction on most subjects.