Earlier quoted context omitted.
If you do, rent the smallest available car. You'll be thankful for it on single-lane tunnels, single-lane (for both directions) roads and so on. This tunnel was fine, a novelty on a relatively long drive. The natural scenery was wonderful.
Or do as I did and just use the buses. Highly reliable IME.
But yes, the buses were good too. The others all used them when they wanted to do something different. (I didn't, as I was the only one willing to drive the car.)