And why wouldn't someone be able to sue successfully?
Deaf people still are unable to properly hear so are more unsafe when driving. They're legally allowed to do so because of deaf people lobbying legislators to force them to be allowed to do so.
For this to be a real danger, either the driver would have to not notice that they suddenly couldn’t hear anything, or the faulty hearing aid would have to pass through some sounds but not others. Both of those scenarios seem pretty unlikely.
Meanwhile, from some quick googling, it seems to be common for completely deaf people to drive; apparently there are devices that can translate sirens and such into visual cues, but such devices are not universally used. To be fair, it’s a bit different if you’re not expecting it or practiced at it.
And with the lack of certification requirement, someone will start cutting corners to make an extra chunk of profit. Just imagine a faulty hearing aid that fails while a person is driving, and the person causing an accident as a result. Good luck suing anyone over that one. Or people mis-tuning hearing aids leading to more damage than before.
Another thing I can easily imagine is unscrupulous manufacturers using banned chemicals to manufacture the hearing aids. We have enough of this sort of shenanigans with nickel allergies in jewelry.