I’ve driven the A9 from the south as far as Stirling to the north as far as Golspie in the north so about 80% of the full length. It’s not a good road for the volume of traffic, esp. heavy vehicles, but it’s no worse than many other busy roads. I’m quite glad that I don’t have to drive it every day.
But honestly, tourists in over-powered cars are often the worst drivers, overtaking dangerously, driving at high speed, failing to observe basic driving safety. Then locals who are drunk or high often constitute the next batch of stupid drivers. These locals may often be quite young, driving their first car and do not have enough experience on the roads. They often may mistake it for a game on their X-box.
There is often no choice to driving the A9 because there is no alternate route, it is the only road in many parts of Scotland. So when it gets blocked, nothing moves. After some accidents, the road can be closed for more than 9 or 10 hours! It’s a serious problem for all users.