Oh, I just remembered a third situation where I normally don't allow the use of Traits for autosucceed:
3) When failing or rolling a certain number (usually an Eye) has special consequences. The best example is Fatigue checks, where an Eye has special consequences, even if not a failure. In this particular example (Fatigue tests) a balance could be found where the hero is granted an autosucceed for
Hardy, for instance, if travelling through terrains of TN 14 or less, but he still has to roll the Feat die to see if an Eye comes out. The triggering of a Hazard is independent of the success or failure of the Faigue test, it's a mechanic where you get two different informations from a single roll.
The text that I find most interesting regarding when to allow or not autosucces is this:
The Loremaster may agree with a Trait invocation to speed
up play, especially if failing at the roll would not lead to
dramatically relevant consequences, or if the action wasn’t
difficult.