We in ServiceNow Professional Services have completed or assisted with a number of CMS to Service Portal engagements for customers with complex catalogs. We work through a relatively common set of problems across these types of engagements and, while the data for this is primarily just gathered from other public sources, we've come up with a guide of sorts for the various issues we see. A big shout out to Bharath Padaki whose blog post here provides the backbone of this guide.