My students have not been typical, and many are scientists and engineers, and motivated high school students. Only one, out of hundreds has taken a formal "ground school." Probably 50 (just a guess) have bought, borrowed or somehow used videos or online coursework. The rest (hundreds), have elected to study on their own. I provide a modified FAA syllabus, my supplemental syllabus, and pointers to a large number of resources. I also provide a reading list. Since I am in the Northeastern US, they read "Weather Flying," by Robert Buck. Even if they are a primary student, I want them to understand many of the things Buck talks about. But that is just an example.
Most of my students meet with each other for review sessions, and if they get stuck on something, I might get a call.
This method won't work for everyone. Many flight schools make money by offering ground schools. Many CFIs are more comfortable with their students getting a comprehensive ground course in a more formal setting. Some students might be more comfortable with a structured classroom.
However, when the students have free reign to explore their material, I find they do better. As an example, one student had a hard time with turbulence in flight as a primary student, and she studied weather phenomena and became quite knowledgeable on lifted index, and so on. This is typical of students who are curious and motivated.
So if one is disinclined to go with a traditional ground school, and your instructor(s) are supportive, from my experience it is not only doable, but can produce superior results. Oh, on the written? I cannot remember any student getting below a 94. It may have happened, but if it did it was unmemorable.
I realize that this answer is not laden with regs and manual quotes, but it reflects experiences having instructed for 40 some years.
An aside for CFIs, the rules may have changed recently, but I normally sign off for the written with my AGI certificate. The record keeping requirement is different.