Dude, that looks like a 47.29 on the flaco scale... I'm walking! Good stuff.
The flaco scale is similar to Corran Addison's river rating scale. Corran gave a run three separate ratings 1) difficulty , 2) danger and 3) remoteness in an effort to better quantify risk.
I think safe is a misnomer here, what you are talking about is how much risk is there. In my mind, rapids of similar technical difficulty vary in risk depending on the danger and remoteness as corran noted. I typically walk drops I know I can run if there is singificant danger or if its remote or both.
So a class "X" river that is the lower risk is typically roadside, close to hospitals and relatively free of wood, sieves and pin spots. A class "X" river that is high risk would be away from the road, far from help and have lots of hazards like sieves, wood, or pin spots.
Also, I think sieves, strainers, and pin spots are more likely to be fatal hazards than holes (big water being the exception). Many holes will release a swimmer, or you can be roped out. Sieves and entrapments are a totally different story. A good example would be rigor mortis on clear creek. Its a technical rapid that ends in a big hole with a fine line around it. If the hole was replaced with a strainer or with a sieve with the same fine line around it, I bet hardly anyone would run if. As is, it gets run all the time because the consequences are typically an ego busting swim and potentially lost gear.