It really depends on what you put INTO it.
My issues with the 3D Visualizer are more based on the 3D Visualiser not being able to do certain things so I can feel better about them working properly, such as sound active modes. But I have a big show where I will have time to test this with coming up.
For me, I've found the 3D Visualizer to be accurate enough for me to get a sufficient proof of concept and comfort level, especially with my moving heads.
I wouldn't give it a 100% rating as I don't think that is feasible with any program. For me, I'd say based on how I use it, I'd feel comfortable saying 95%.
Really, most of the issue is how much time and effort are you willing to put in ahead of time to your stages and the accuracy of your mountings and placements? For me, I use the same generic room and it has worked fine for me, because most rooms I need to concern myself about are square.