I often fly on 016 ticketed UA/CM itineraries. It's rare that I can get digital boarding passes on either UA or CM app. Almost always have to get printed passes. If originating on CM they will sometimes give me passes all the way through, and sometimes I have to go to a temporary UA transfer desk in PTY to get them. Going the other way, United will usually give printed passes all the way through, but I have to fuss through numerous kiosk screens or get to an actual human agent.