Are you using any sort of damage mitigation at all? Spells? Any armor? I don't often see bandits dealing out 150 points of damage in a single blow.
At the time, I had light armor boots and bracers (both improved Scale) for armor, and a set of robes with a circlet. Two-handed power attacks from enemies are considerably deadly.
But yeah, luck in fights is perfectly appropriate.
Luck would be fine if it was something like crit damage on a strike, +/-5 damage, or something minor like that. But here, it's essentially a difference of being nicked by a weak regular attack, or getting instantly hit with a full-on power attack with no ability to dodge it or reduce the damage.
But the claim that your block should be essentially something that can't be gotten around just doesn't make sense.
Blocking doesn't prevent all damage, but it does mitigate some of it. You can't rely on blocking to keep you alive indefinitely (you will need some way to recover health eventually if you can't kill the enemy), but it shouldn't be completely ignored, either. Being at "low health" isn't too descriptive here, but given what I've seen with my character, the complaint over the way finishers are applied to the player is more than justified.
Once in a blue moon it makes sense that sheer bad luck will change things up. Skill should be the prime determinant of a fight's outcome, but that doesn't mean no luck at all should be factored in.
Luck is fine when it's a reasonable variation. But going from 'easy win' to 'instant death' on Luck alone is far from reasonable, IMO. You don't see it coming so there's no way to even try cutting your losses and running, and there's nothing you can learn from it because you don't have to do anything wrong for it to happen. For me, it does nothing but result in frustration as I can lose a good bit of progress through no fault of my own. It doesn't teach me to "get better" or tell me that the enemy is too strong, because neither has to be true (and if it is true, it's not the killing blow that tells me that).