The major limitation is that it is SLOOOOOOOOOWWWWWWWWWWWW.
Did you get that Google? You have written the ONLY program that manages to bring my Macbook Pro to it knees. Well done. I have spent maybe 30% even 40% of the last 5 hours of my life watching the spinning beachball of doom.... Honestly if Bonzai gets a little bit more stable (i.e. doesn't crash every 5 minutes) I'm gone.
Your latest release, 7.1 took nearly a year and in my opinion is only Beta quality. Without the 3rd party plugins SU would be dead and I'm sure 99.9% of your users wouldn't bother due to the default tools being old hat.
The model I'm working on is a simple house, on a SMALL site. But place 2 trees and four plants and voila... beachball. Orbit, beachball. Pan, beachball. Zoom, beachball. Move component, major beachball.
How about this for a new tag line?
SketchUp Pro, Waiting is half the fun.