So says International TechneGroup Inc., a firm which translates large numbers of CAD models between formats. Older surface modelers such as versions of Catia before V4 use the approach of sewing surfaces together to create solids. Using the same techniques with newer parametrically based packages creates difficulties in IGES or STEP translations, but also in any sort of downstream efforts that employ the solid model as a basis for operations such as FEA or manufacturing operations.
Here are a few examples of model quality problems that ITI personnel have uncovered, and practices that would have avoided the difficulty.
Some early IGES history
Early versions of the IGES spec were based on the Boeing Database Standard Format (DBSF) which was heavily influenced by the CAD systems in use at Boeing during the 1970s: CADDS 3 and Gerber IDS. Both used simple geometric elements and basic text and dimensioning abilities.
Ground rules for adding to the IGES specification were that an entity had to exist in three major CAD systems before it would be considered. Thus anything unique to a given CAD system would be excluded. Items in a CAD system that were special, probably also constituting a good portion of its competitive advantage, had to be converted to a different concept in IGES or be dropped altogether. This practice sheds some light on why useful features found in CAD systems have been slow to find their way into IGES.