A negative, compared to government funding, is that the economy has lost mobility of labour. Getting a full scholarship requires the child to be enrolled from K-12. Loss of mobility might be a good thing, in that it prompts people to improve their local economy, rather than run away. Alternatively, it could lock people into a form of serfdom, unable to move to better their situation.
I'd advocate that a voucher system, whereby the government gives each student a voucher for the degree of their choice, would be preferable to the widespread adoption of "the promise".