As stated earlier the authors keep the copyright of their work; additionally authors are paid a royalty for sales in all formats. ProQuest is extensive and global archive. If the university does not participate in ProQuest Dissertation and Theses Dissemination program, one can still submit the work. 

I don't find any real disadvantage.It may be mentioned that  ProQuest submission is equivalent to e-repository submission, and there is no peer-review process.

I also suggest that you make the effort to talk to colleagues about the choice of the best serious scientific publisher. Publishing houses like Springer, Wiley, and several others have quite many book series, which means that you can target the right audience by choosing the right series - if, of course, the quality of your work is sufficiently high. You may perhaps also send along some recommendations from your thesis committee members, supervisors, et al, so as to convince the publisher that it would be worth while for them to publish it.