Given the right time of year and a VERY little pigeon work to make sure there is some pointing instinct, I run them on wild birds exclusively as pups. This is a greater than forty year habit at this point - no libbies until they've had a maximum amount of wild bird experience and then broke on pigeons. Obviously we are forced to trial on liberated birds in many places so you must introduce them and finish them or correct them on fluttering birds, etc. I run very few Derby trials for this reason as well - dogs aren't ready for those libbies and catching them sure won't help me finish the dog. I am after the adult dog and a long trial career if so blessed - not just a flash of Derby glory.
I have NEVER had a dog flag on libbies or refuse to point them, etc that you hear about. I attribute those failures on libbies to overexposure and mistakes made with those pups when young with those liberated birds. Wild birds to learn to hunt and find game and then pigeons to staunch the dog. NO libbies ever until broke.
It is why we go north, west and then south to expose dogs to wild birds. You can likely do enough with pre-released coveys and feeders to get this done but if you are depending upon throw down birds and Johnny house releases, I think you're setting yourself up for problems. At the very least, you're not getting the maximum out of the dog.
Dogs learn a ton from the wild bird experience including where birds live and how to use the wind, not to crowd them, etc. Whenever I've seen dogs go stale from overwork on pen birds, the universal cure is "take them wild bird hunting".