I have studied the Forbes ranking over the past few years. While reasonable minds can certainly differ over the use of an ROI metric, it seems to have at least some utility as an evaluation criterion when used in combination with other criteria.
The greater problem with the Forbes metric is that it just isn't executed very well. The calculation is profoundly sensitive to specification so that even even small changes can cause major changes in the ranking of programs (in other words, it exhibits high levels of ordinal fragility). When I recreated their rankings, I found that schools were often separated by less than 1% net benefit, a difference that was so small that it was swamped by small changes in specification.
The are two major omissions-
1) A rigorous model would incorporate taxation at the state and local level. While this might sound trivial at first, consider that some states have no state income tax while others have tax rates greater than 5%. Over the course of five years, the net benefit (after tax) of students in low tax regimes would be several thousand dollars higher than that of students in high tax regimes.
2) A rigorous model would take into account cost of living for different regions of the US. Depending on how one measures COL, one dollar in a less expensive region like St. Louis could easily be the equivalent of $1.50 or more in other parts of the country. Adjusting for COL can cause programs to shift several places.
Another significant problem is that the calculation is biased against public universities that offer significant savings for in-state residents (recall that the calculation is based on out of state tuition). When I adjusted the figure for UCLA in 2003, the school jumped from a rather mediocre #18 (including intl. schools) to a far more impressive #5. The same basic pattern can be observed with Texas and UCB. As an aside, it is a bit odd that they use the current tuition to compare schools when it would be more logical to use the tuition of schools at the time that the cohort was attending school.