I studied a PhD in this field (dropped out after one year).
There are useful prototypes out there ("useful" as in someone with motor neurone disease moving a computer cursor on a text to speech machine), but all of them use invasive, penetrative electrodes that fail after a matter of months as the body rejects them.
The less invasive prototypes have seen no real breakthroughs in the last decade and a half. Only a small percentage of people can use them with any real accuracy, nobody knows why and frankly nobody's doing the research on it.
All the grant money goes to people throwing random ML models at standard datasets and patting themselves on the back when it gets a 2% more accurate fit on one (and only one) set.
Couple this with the fact that technology relying on other biosignals is yielding amazing progress (mainly the translation of voluntary muscle/tendon movement into control signals for a computer, like what Stephen Hawking had but more recently prosthetic hands with dinner and finger control), I suspect that most of the research into BCIs will be put on ice until we have a more complete understanding of how the brain works or can no longer make progress with other control technologies.