I did NOT want to go to my flagship in my home county. Could not afford OOS/private schools. Went there and it was a world apart (and the world came to me, including people from NJ). I was a NMS (single award) with top stats and did whole Honors Program thing at a good school.
I think you could approach your D with short discussions about academic peer groups. She probably is used to being in classes with other top students. She likely has been in some classes with the average students (I was in some in HS due to scheduling difficulties). She may love a campus but she needs to consider the people she spends her time with. You can point out to her how she is used to her friends “getting it” quickly and functioning at a certain level. How will she feel in a class where she is consistently ahead of the other students?
Another factor can be what is expected of students and covered in classes. Will as much material be covered in Subject 101 as in the flagship schools? An example- a friend taught a couple of years at a state school (wasn’t a good fit for her) where less was to be taught in a semester course than she had TA’d under the quarter system.
Your D may assume that all colleges teach at the same level and not even have thought about the academics much. I assume she has taken AP courses- those are theoretically college level but only average college level. Her top flagship U possibilities will offer much more for a given course, especially at their Honors level. AT UW-Madison, for example, students who pass AP calculus still most often start with first semester calculus, even the regular non-honors, sequence. They simply won’t have covered as much material and often flounder if they start with the second semester.
The above are some concrete examples. Sometimes we get used to being with people at our level and forget that most are not as academically sharp (pardon my attempt at euphemisms). This sort of argument/point may open her eyes to what you are talking about.