Cometic Head Gasket

Personally I won't run Cometics unless I really need to for design considerations (like the 383 or 273 builds where the standard stuff gives away too much.
When they are used - the surface needs to be very smooth relative to "normal" stuff. You can use coppercoat, and it's a good idea to remove the rivets, but the surface per Cometic needs to be 50Ra or smoother. Edelbrock heads ship with around 55 which is why some guys have leaks and others dont. It takes a modern high-speed cutter to get really smooth surfaces. In comparison the typical felpros and composites can seal from 50-110Ra. If your shop doesn't have a profilometer then they don't really know what level they are getting anyway.
I'll add - if you are not getting the correct compressed thickness you should review your block and head prep, depth of bolt holes, dowel pins, condition of the fasteners, etc. If the gasket is hit with the right load in the right sequence it will squish to the correct thickness.