Shell ejection CGI always looks though. No bolt cycle, ammo belt feed, recoil e.t.c. Lots of limitations. CGI won't be fully replacing blank modded firearms in film and TV for a while I don't think.
Something that really bugs me in films with miniguns is 90% of the time they mess up the sound. It's one of the most beautiful sounds on Earth and they dub some piece of sound effect on it.