You've hit the nail on the head. There were actually impressive trials done wherein researchers used invasive sensors that penetrated the brain tissue itself a

You've hit the nail on the head. There were actually impressive trials done wherein researchers used invasive sensors that penetrated the brain tissue itself and directly measured neutral spikes. The idea that ML could magically classify signals from sensors above the dura/skull was laughable. Your signal (and therefore training data) is affected by basically any thought of action the user has, and is then spatially low pass filtered by a big fat slab of bone and flesh. Imagine trying to train a "cat recogniser" by showing it low-resolution pictures of a random location in a room where a cat is. It was a joke then and I suspect still is now.