In a couple instances I've taken existing structures in our library and modified them in the block editor to match jurisdiction-specific dimensions. I then create (copy) a new style for the new structure. This seems to work well and is easy, it at least looks fine on the plans, but... an I under-thinking this and missing something important? For example, should I be creating all structures from scratch?
Nope, I don't think you are missing anything.
While Civil 3D is marketed as a BIM product it doesn't even come close to being a BIM product. Even if you do model the structure correctly in Part Builder showing structures in profile view aren't even projected correctly if you do the outline of the structure as the style. Civil 3D assumes you look at it at one direction and doesn't reflect the rotation of the structure in the profile view. So in short Civil 3D sucks big time as a BIM product, Autodesk doesn't care to improve Civil 3D into a BIM product and just markets it as BIM product.
What gets me mad is that if you create Outside Drop Manhole that is part of the Civil 3d part builder tutorial Civil 3d is unable to display this part in profile correctly. You can display is a solid object, but not as a boundary. It would be nice is Autodesk would tell you this up front. I spent a few hours going thru this tutorial thinking this was a useful custom structure.
PG