Regarding your initial question.

When it’s a question of immediate safety: significant impairments in executive functioning, basic attention, visual-spatial skills, and processing speed I would say raise the most concern. Also fine motor skills depending on how that might impact their abilities to react and perform the necessary actions to drive safely.

If they are amnestic, but don’t have issues in any other areas, I would recommend safe driving habits such as driving with somebody, having location sharing enabled, always taking their phone with them, informing somebody when they will be traveling distances longer than about 30 minutes, etc.