One criterion, advocated by Khun Angkhana Neelaphaijit among others, is the likely effect on those whose rights have been violated or are at risk of being violated.
In cases where a public campaign against e.g. an illegal detention can reasonably be expected to increase the risks that detainees will be ill-treated as a form of punishment, then perhaps we look for alternative ways of achieving the detainees' release. In cases where people have deliberately courted arrest, to highlight the inequity of a certain law, for example, perhaps we would not campaign for their release, if that is not what they themselves want.
A strategy that says every known human rights violation must be the subject of a public campaign could, in what is of course a minority of cases, risk a negative outcome.