Here are a few more traits of some custom actions not mentioned:
Some of the custom offerings like Surgeon or Stiller offer larger actions with magazines large enough to fit cases like the RUM or 338 Lapua or even a chey-tac with enough space in the magazine for seating VLD style bullets. Some have integral picatinny rails. The bolt bodies are also larger with more steel around the case head. To me those customs fit a niche that an altered Rem or Win action cannot fullfill.
