No one who has ever been *completely* brain dead has ever come back to tell us about it. If a person survives their experience, they had some brain activity. It may barely register on machines, but it's there. No exceptions.
That said, we don't ignore NDE's, not at all. We simply don't regard them as proof of anything, any more than some guy having a religious experience while on LSD would register to you. We do, however, chuckle at the irony of being called closed-minded, when we are the ones allowing for any number of explanations that need not absolutely require *one specific* type of afterlife phenomena to be plausible.
We are the ones that say there are other possibilities. The brain, anesthetized or in a coma, is still picking up sensory information from the body, even though consciously for the most part is not present. The brain, as it always does, even when awake, processes this info in unfathomably complex ways, so as to package a neat story that it tells your mind during those brief periods of semi-consciousness. When you awake, you regard that story as a memory, and being superstitious, you tell yourself that it was a first hand account of things you couldn't possibly have seen (because your eyes were closed, and also you were functionally dead).
The reality is that the feelings, sounds and smells your brain picks up are relatable to things you've already seen (visually) before. Furthermore, there are really only so many things that can happen in a hospital setting, so it's not difficult for your brain to paint a picture of things that actually happened. Very few people claim to have seen, say, a clown walk into the adjacent room and silently form balloon animals for their daughter while waiting for them to awaken. What if that happened? Why didn't they "see" it? Because there was no sensory input to form that picture, because it happened outside of the range of their senses. The things they tend to "see" are along the lines of things one could expect to actually happen (tears, low talking in the corner, the doctor announcing the surgery was a success), along with the story that the body and brain help to create about what was happening to them.