I'm hearing a lot of Cons related to how the neck knife feels swinging on your neck, as well as Cons related to deploying the neck knife. First of all, as I've mentioned before, deploying a neck knife takes a little bit of repetition and "muscle memory", just like any other similar task. It takes a little practice to get good at deploying and using your folder, your firearm, your toothbrush, or anything else.
If you use the cord or chain around your neck knife to pull it up out of your shirt, it doesn't matter what you are wearing. You could be wearing a t-shirt, a collared shirt, a jacket, anything, but as long as you can reach your neck and grab your knife cord, you are good to go and can retrieve the knife. Again, this takes a little practice to get good at it. Give it a shot, and you might be surprised.
As far as the neck knife swinging around your neck, what I normally do is give the chain or cord only just enough slack to get over my head. If you give it too much slack, it will swing and swing. If you have it tighter around your neck, it is easier to retrieve and won't have as much play.
That's all I can say. I love my neckers. I always have a lot of stuff in my pockets, and getting anything quickly out of there is usually a nightmare. I don't work or live in an environment where belt carry is appropriate. Neck carry for me is perfect, because it allows me to discreetly carry a fixed blade knife.
I hope that helps. If you don't like it though, you don't like it. I don't really care, all I know is it is a system that works out well for me!