To be assertive when you are right is a good thing . To be assertive when you are not so sure is where it becomes difficult.
I do not mind anybody (and that includes my wife !) to be assertive once in a while if in the end he/she is proved right.
As to controllers telling pilots what to do when they do not feel at ease with what they see ( the original question basically) this also depends a lot on the controller education and his position in the social ladder in his country. The UK and US might be fine , but the examples taken earlier of Cali, Teneriffe (in 1977) , Bahrain , etc..are prime examples where controllers did not have the training, nor the authority to be assertive to Pilots.
If one take Brazil , and the recent accident there, the Brasilia ACC controllers involved were all military with rank sargeant at most.
But in the present and certainly in the future, not acting as a controller when you spot something wrong will get you most probably in jail if it ends in twisted metal.
. I very recently followed a legal seminar on this subject, and everyone present , the judges, the Barristers and the Aviation States prosecutors ( they were a few of them including the UK one ) all said the same thing : Controllers can no longer hide behind books not to execrcise their " duty of care " . In other words , controllers in case of accidents will be judged ( and sentenced ) for what they have not done rather than for their direct actions.
Now, when I fly and when I get ( very seldomly because I am so good
) a " direction " or a " correction " by a fellow controller, I follow it first and eventually, if I am not happy with it, discuss it later in the bar.
