The real design can not be from the low to the top, must be the first overall, and then remove the parts, that is, top-down. If you use the bolt, the bone is still first have the whole combination, and then select the bolt, or top-down.
ug completely replaces AUTOcad, no problem at all, otherwise it would not be sold so expensive.
But instead of AutoCAD only in doing the real sense of the design will feel his practicality.
In a word, ug is suitable for design, not drawing.