I think its also important to realise that the idea of releasing captive bred animals into the wild intentionally as part of population restoration measures is a very new idea.
Sure animals were released in the past, but it wasn't really with the intent of bolstering wild populations and as a result wasn't studied after the release much. So I would say there's still a huge amount that we have to learn, though I think its very valuable information as well.
It forces us to study animals to a much greater level, to really understand their language, their learning process, their survival and how they live in the wild. It really highlights that humans are not alone in having a learning phase and firmly kicks the "animals are all instinctive" idea out the window.
I also think its important to identify that re-introductions are only as good as the environment they are released into. If threats such as population isolation; poaching; habitat loss; pollution; excessive predator pressure; climatic change et c.. are still present then reintroductions can only, at best, prop things up. Of course there's a difficult part where you have to decide on if you keep reintroducing and hope that you can prop things up for long enough to maintain a wild population whilst you continue to battle the other pressures; or if you run the risk of losing the wild population but maintaining a captive one for a longer term release in the future.
For zoos, who have display as one of their primary features, release is also difficult because its a totally different approach to captivity. Release individuals generally have to be kept away from the public, in different kinds of enclosure and have to be, as said above, trained and prepared for life in the wild. You also often need a captive site in the release area or at least the release environment in order to prepare those animals for release into the area where they are to live - especially if your release individuals are being raised hundreds or even thousands of miles away in different countries and climates