My workflow is to leave Maya as quickly as possible. That means opening the dataset in Maya, maybe flipping some objects that have wrong normals and that’s it. Write to fbx and import that in Max.
Welding works just fine using the ProOptimizer. Smoothing is never ever to be touched with CAD data. No need for that, as the data you import has to look good in the first place. If it doesn’t, something went wrong during export/import. Smoothing on CAD can not work by design, and I am afraid would be the same in Maya.
Artifacts often appear when you do certain operations on the wrong type of object. Unwrap UWV on edit mesh always breaks normals. You have to convert to editable poly first (select objects to convert, then right click, convert to edit poly). Operations like unwrap and reset xform should work just fine now. Not sure about shell, that thing has its own problems. Most of the problems with artifacts arise because there are these two different types of meshes. You have to know when to use one or the other. Same for flipping faces, both types have a function for that, but they do different things. One flips face normals, the other vertex normals.