Vader took a full-on blast from a Sith Lord, who very likely inflicted a seriously vicious critical injury with that sustained barrage. To say nothing of having just been in a major lightsaber duel with his son, and likely lost the bulk of his wounds during that fight now that Luke had gotten his 'saber prowess up to snuff in order to actually challenge Vader in a fight.
And while his life support system was badly damaged, Vader still survived that blast, and really only bought it due to narrative concerns since he was pretty much an invalid without functioning cybernetics. Could very well be that he got tagged with a "Bleeding Out" result on top of having his Wound Threshold exceeded by that barrage of Force lightening, and kept suffering more critical injuries (such as "Hamstrung" to lose his free maneuver and "Knocked Senseless" to lose his action as well). Again, with Palps having high marks in Willpower and Discipline as well as a hefty Force Rating, it'd be a trivial matter for him to get past Vader's presumably beefed-up soak value, and then use any Advantages to trigger a critical several times over.
As for the Mace Windu example, it wasn't Force lightening that ultimately did him in... it was the sudden stop at the end of a very loooooooooong drop that put the final nail in his coffin.
A character with a Willpower of 3 and one rank of Discipline isn't going to do much with Unleash, any more than a character with one rank in Lightsaber and a governing characteristic of 3 is going to accomplish against a serious threat. If anything the guy with the lightsaber is just hoping for that one success against a major-league adversary and that the target didn't load up on Parry.
Protect/Unleash is definitely on the mystical side of Force powers, geared for characters that are built to be Force wizards. And with the requirement of Force Rating 3, a Force wizard build that doesn't have at least a Willpower of 4 and 3 or more ranks in Discipline is doing something seriously wrong by the time they purchase this power.