It is a crapshoot, to a certain extent. There is risk involved in any move, but obviously, some moves are much riskier than others. It's very easy to judge a move based on hindsight, but IMO, that's usually unfair. It's much more difficult to anticipate a move going in.
Often, at the time of the move, the move is considered to be a great move by most fans and analysts. If the move does not pan out, for whatever reason, I don't understand all the criticism the FO then receives. Just because a move doesn't work out doesn't mean that it was a bad move. As you said, there is no certainty.