I'm not sure I understand the question, you're asking about pros and cons of mini itx cases but then say you're buying the bitfenix colossus which is a full atx case. If you're getting a full atx case then pros/cons of mini itx cases doesn't apply.
In general terms, mini itx have size as an advantage if someone wants a pc with a smaller footprint. Both to save space and easier portability. Depending on the case, size is a concern most likely requiring water cooling if trying to go with a higher performing cpu since air cooler choices become quickly limited. Motherboard choices are more limited both in models and features (less fits physically on a smaller board), you may not be able to crossfire or sli, it may require a smaller than...