This rings true for us. The school ticked so many boxes (size, FA, culture, student:teacher ratio, etc…) Location wasn’t limiting to us as we were going to be flying to any of these schools, but it did not offer the sport that my son loves. So we moved on. That was the case at a few otherwise amazing places.
While we had not heard of it before beginning to look in earnest, it did come up in several searches rather quickly once we began filtering for our criteria.
I haven’t looked at their college placement, but I do think that some of the super elite schools attract people who are very brand conscious and very hooked. So their continuing on to equally “elite” colleges via said hook is not a surprise. Nor is it an indicator of how well said school will prepare your student for the admissions game and beyond. In fact, your student might benefit from having fewer of those students in their fellow applicant pool senior year?
It sounds like you have a fantastic “match” with Mercersburg. Enjoy it. I wouldn’t think too hard on this until March 10. If offered a place, that is a good time to get granular about college placement. Asking for too much detail prior to acceptance might give the impression you are looking more for reputation/prestige than fit. That will not enhance your daughter’s likelihood for an acceptance at any selective school. 22% is a very competitive landscape.