Duty finders primals are basically luck of the draw. Ifrit and Garuda should be fairly reliable in terms of success rate. That being said they do fail. Duty finder Titan, on the other hand, is pretty bad most of the time. There certainly are duty finders Titans that succeed but they are few and far between.

If you're going for relic and don't have an FC or friends you can do it with I would day use duty finder up to but not including Titan. Try and get in on an Onion or Rodeo titan carry as those happen from time to time.