Thanks for the quotes.

To begin with, there are significant risks of medical complications—including infections, electrode displacement, hemorrhage, and cognitive decline—when implanting electrodes in the brain. ... One study of Parkinson patients who had received deep brain implants showed reductions in verbal fluency, selective attention, color naming, and verbal memory compared with controls.

I was aware that BCI would be dangerous, but I wasn't aware that current BCI, with very limited bandwidth, was already so dangerous. As I said, one could try only interfacing with the surface of the brain - the exact opposite of deep brain stimulation - which is less invasive but does massively reduce options.

Not only can the human retina transmit data at an impressive rate of nearly 10 million bits per second, but it comes pre-packaged with a massive amount of dedicated wetware, the visual cortex, that is highly adapted to extracting meaning from this information torrent and to interfacing with other brain areas for further processing.

Outgoing bandwidth, OTOH, is only a few bits per second. Better to pick the low-hanging fruit.

Coincidentally, I ran into a paper Reconstructing Visual Experiences from Brain Activity Evoked by Natural Movies which seems to claim to extract large amounts of information from the visual cortex via fMRI:

The decoder provides remarkable reconstructions of the viewed movies.

Although I don't know the specifics because of the paywall.

Even if there were an easy way of pumping more information into our brains, the extra data inflow would do little to increase the rate at which we think and learn unless all the neural machinery necessary for making sense of the data were similarly upgraded. Since this includes almost all of the brain, what would really be needed is a “whole brain prosthesis–—which is just another way of saying artificial general intelligence.

As I said, I think taking information out of the brain would happen long before this scenario. But in the more futuristic case of an exocortex, there would still be a period where some parts of the brain can be emulated, but others can't, and so a hybrid system would still be superior.