Well, I went through two flocks before finally succeeding in developing a system that kept them safe. Even then, I had a few more losses before the system was perfected. So I don't think it's useless to try if you really want them.
However, I would think carefully about what you've learned so far and what else you need to know in order to be successful. Take the time to read Storey's Guide to Raising Ducks and hang out on these forums for a bit. Look at your set-up and decide what you need to do to make it safe for your ducks. If you're worried about human theft, maybe your pen needs to have a padlock on it. Regardless, you need to have a pen that is predator proofed, made with sturdy materials and completely contained.
So, just examine what you have so far and learn what you need to do to improve. Then I say, if you really want ducks, go for it. And good luck!