On what basis do you say that meta-analysis is not credible? From my stats background, (but without any direct experience in meta-analysis) I would say that they could be problematic because the N is very small and so have a lot less to work with, and more temptation to use fancy statistical techniques with hidden assumptions. On the other hand, what do you do when you have 50 studies of varying quality, no one of which is has such a compelling methodology as to eclipse the others? In that case it seems like meta-analysis is the least bad option.