Tuck almost always does great in rankings. I believe they have held the top spot in most WSJ rankings (which really look much shocking than these). Tuck is an awesome school and their average salaries are on par with HSW . As I see it, the only drawback for Tuck is its location. Granted, it's in the middle of nowhere and get super cold, but for those that have decided to attend, it's tough to beat. It's not for everyone, but for the criteria of this ranking, Tuck is obviously strong.
ROI is definitely not the most important criteria for business school rankings, but it does have value. Harvard and Stanford grads to have the highest salaries, but they really should because they skim the cream of the crop during admissions. ROI can tell you what the other schools do to improve the standing of their students.
As far as Darden, all my books/magazines are in boxes right now, but I do know that Darden grads have higher salaries than peer schools such as Duke, Cornell & UCLA; highest among the elites if I recall. So, when you talk about ROI, Darden usually does very well. Regarding UCLA & Berkeley, if I recall, their employment numbers were down compared with other elites. That's going to hurt in an ROI ranking.
I guess the summary might be that Harvard and Stanford get the best students to begin with, and naturally these people will later command the highest salaries, almost irrespective of their MBA educations. On the other hand, the mere mortals admitted to lower ranked schools stand to benefit more from what their schools can deliver. That's ROI.