Are there meaningful downsides to a "guaranteed" (graduate) admissions program? One  school I know of, University of Pittsburgh, has an early admission program to its graduate schools (law, business, medicine, and others) for qualified juniors. That is, students can apply to the program in the second semester of their junior year, and if admitted, can use the first year of the graduate program as their "senior year," in terms of credits needed for their bachelor's degree.

More to the point, this school has "guaranteed" graduate admissions for outstanding high school students, to which one of my smartest high school classmates was admitted. She had outstanding grades (4.0) and board scores (1500), and was "guaranteed" a place in the medical school upon matriculation. The minimum requirement was that she had to take a "general science" major (biology, chemistry, physics), and earn a 3.75 GPA, but given her high school achievements, this was not in doubt. (She now has a thriving practice.)

For someone that knows what they want, are there any meaningful downsides to "enrolling" in a graduate program (medicine, law, business), right out of high school?
If you are sure that you want to be in a particular profession, say law, then these programs can provide a bit of a short cut. However, if you change your mind along the way you may wind up with an "unusable" education as it may be missing critical elements needed elsewhere. 

However, if you can change your mind and still have a good path forward, then there should be no real downside. In particular, if the intervening years include a real undergraduate degree in a meaningful field, then there is no reason to actually finish the professional program. There isn't a contract that you have to fulfill. 

In some ways this is just a marketing stunt by a school who wants to increase applications of good students. There are no guarantees on either side, I'm sure, as the student has to fulfill the requirements stated to move on to the professional portion, and the student is free to leave at any time that they feel their needs aren't being met. 

But a program that is so specialized that you don't get a good general education along the way would, in my view, be a mistake. A mistake for the individual as well as for society. A program that only makes technocrats can turn out people who are actively dangerous as their world view is too narrow. 

Moreover, if you want to later extend your education beyond the professional training you get in such a program, you may find that you don't have the needed background. That would depend on the details, of course. 

Actually, I also question the wisdom of encouraging people of that age to 'lock in' to a particular profession. A lot of changes happen in the lives of 17-18 years olds. As a parent I would discourage such things in favor of a more general education in which they could put off such decisions until they have more perspective. I recognize that not everyone feels that way, of course. If a kid wants to be a doctor simply because both mom and dad are doctors he/she should really look around first. (I feel the same way about academia as a profession, in fact.)