You are correct in your assessment. Every dog is different, so not every dog will figure out that when it's off, they can't be corrected. E-collars are like any training tool, and they will work miracles with some dogs, and do nothing with others, and a whole lot in between.
My Sophie is about a stubborn as the come. I used an e-collar with her, and she learned to ignore it when she chose too. I conditioned her as recommended by people here, video's I watched, and info that came with the collar. Once she figured out that it wasn't going to kill her, she would just put up with the stim and do as she pleased. I'd press the continuous stim, on max, and could see her twitching, and she'd still ignore me. After a month I gave up and really started using NILF 100% of the time. She's still stubborn, but is doing much, much better.
I did put the unit on my arm set on max and nicked myself with it before I ever used it on my dog. It was unpleasant, but I didn't find it painful.
I'm sure others will comment. I think that depending on your situation, and the goals you have for your dog, using an e-collar could be very beneficial, or a complete waste of time. It's up to you to decide.