The relative risk of driving while having ADD/ADHD (accidents/km with / accidents/km without) is around 1.54 [1]. This isn't
horrible, of course, but it's still
bad.
Non-disqualifying visual impairments have a relative risk down around 1.2, implying a surprisingly low cutoff for a visual impairment being disqualifying. "Severe behavioral problems due to ageing (dementia)" have a RR 1.45. A category covering epilepsy, narcolepsy, and other conditions causing sudden interruptions/disturbances in consciousness is at 1.84, and those conditions require medical sign-off in every country I checked. Diagnosed alcoholism amortizes to around 2, and that'll get you a breathalyzer or suspended license right quick.
The cutoff for being ticketed is is observed to lie around a relative risk of 4, which is where cell phones come in. The cutoff for being arrested on the spot, being legally drunk, is a relative risk well above 10 and probably up around 40.
Interestingly, almost nothing is as bad as being a teenage male, which carries a relative risk around 7. Teenage females are around 5.5.
That all said... while we can certainly complain about rampant misdiagnosis - I wish I had a dollar for every time someone told me that I'm just addicted to stimulants - my opinion is that this particular rationale for disqualification is absolutely justified. There's a reason I don't drive.
[1] https://www.toi.no/getfile.php/Publikasjoner/T%C3%98I%20rapp...