How challenging is it for a person who has been deaf for let's say 20 years to suddenly regain hearing?

I can't say anything about the specifics of this treatment, but in terms of their ability to fully benefit from hearing, it would depend on when they became deaf, and the severity of their deafness.

If they were born deaf, or lost hearing as a young child during the language development stage, then it would probably be a long adjustment. Things would just be noise and it would take a lot of training to distinguish sounds, speech, etc. And unlike a cochlear implant, you couldn't just take it off to give your brain a rest.

If they had hearing loss later in life, or some residual hearing, then they probably have a better chance of re-adjusting to hearing.

If they've been deaf from infancy, basically the entire hearing center of the brain is non-existent. So they'd be hearing sound, but processing it into meaningful content would not happen, if at all. So basically, its like having a cacophany of sound that you can't filter and process...

As for others, one thing hearing people, particularly monolingual hearing people, don't understand very well is that hearing != understanding. Just because you hear a sound doesn't automatically equate to it having meaning. The default for many people is to just SPEAK LOUDER and slower, which does not help in the vast majority of encounters

Really challenging. In some aspects it can be worse than to regain vision.

If you are not accustomed to sounds, they can be annoying, and may make you feel tired. The same can happen with vision, it is just too much, but you can close your eyes, and shut out vision stimuli. You can't do that with hearing. At least if you regain hearing with normal sensitivity, you can be overwhelmed by sounds of your body.

It is easier with implants, which can be shut off.