How much do your players trust you? I've been playing in my current campaign for over a year now. Last session, my character was (narratively) channeling his lifeforce into an important spell, and finishing the spell was, in the DM's words, "the last thing you know, as you slump to the ground, dead." After a moment of "Waitwhat- what?" I (being familiar with his DMing style) realized that the DM wouldn't have just killed my character without a warning signal unless he had something planned. As it happened, another character intervened to revive me before whatever plans he had came into play, but as it was I had enough trust in the DM to sit helplessly during a tense and dramatic scene rather than protesting my character's unexpected demise and causing the game to lose momentum.
Consider also the implications of declarations versus dice. Rolling the dice, even with impossible modifiers, has very different connotations from declaring that "this happens". Later that same session, the DM made a bad call: As a result of necessary situations, means, and motivations, something terrible happened to a valued NPC offscreen, without a dice roll. This struck me as immensely unfair and I had a several-hours-long talk after the game with the DM about it; he later agreed to make it a roll (with absurd modifiers, as it was a beyond-Epic villain's attack; the result was something like 58 vs. Fortitude). And when it was resolved, the party bolstered the NPC's defenses with Fate points sufficiently to mitigate the attack, which left that plot arc ending on a much more palatable note.
So there you have two different examples, one where a dice roll would have felt vastly unfair when a declaration was acceptable, and one where the reverse was the case.