Krisskross' remarks are accurate, and very politic too. There are a large number of FO who are not promoted simply due to politics, notably the majority of the Dutch.
If an FO ever makes a critical comment in the crew room, or to a more orange captian, it gets reported straight back and that will be at least a 2 year deferment, probably permanent. The same has happened to an FO who refused to go into discretion.
While the assessment process is pretty fair and objective, it is very clear that getting onto the assessment program is entirely based on the fancy of the base captain.