Pretty much. I don't know if I've ever died as a result of a Fallen One type attack, not sure if I've even had a single party member KO on account of one, so they've largely lost their effect on me. However, back in the SNES days they definitely gave that adrenaline rush of gotta use that megalixir right away. I would say those type of attacks work much better in an ATB style gameplay than in a strictly turn-based game. In turn-based they probably have to be paired with some other threat either status effects or debuffs at the same time, limited enough healing resources (either in quantity of curing items or how much you can cure per turn) to cause some concern even when you have a chance to think about your actions, or multiple attacks per turn for the boss. That last one could be too cheap though if they can actually randomly just do an unblockable HP to 1 attack (as HP to 1 attacks usually are) followed by a full party damaging attack with no chance to act in between. It shouldn't be possible for a final boss to do something that will result in a game over regardless of what the player does. Therefore, that last scenario should only be possible in games where a player can prepare for that eventuality with Life 3 type skills that raise you in case of death.

As for optional superbosses, they should be legitimately difficult or else what's the point in having them? That said, I don't generally care that much about optional superbosses and if I find them too frustrating I probably just won't beat them. That's fine with me. Leave them for the players that actually care about that stuff. As long as they're not required for the story, they can be as hard as the designers like.