My blocking is sufficient that if an enemy hits my shield I take around 5% damage. I also have rapid reflexes so if an enemy power attacks while I'm blocking I am at least aware of it. I've been playing with a very cautious approach, generally hiding behind my shield until I'm certain I have a window in which to attack - and of course being a paladinly character I can use restoration to restore my health (Which I do often, I try keep it above 66%).
Trying to do the first companion quest was a nightmare; i got impaled by the silver hand no less than 17 times. Ok, so maybe my character is just weak you say. Well here's the thing, without my shield up, they hit me for 20-30% damage, if a power attack got through it could hit for 60-90% damage. With my shield up they hit me for around 5% damage per hit. Now, remember, there's no way for them to power attack me without noticing when my shield is up as the rapid reflex perk slows time for me, and their basic strike is not enough to outright kill me. Therefore the only way they can 1-shot me from 90% health is I were to be a complete numpty and let them land an un-mitigated power attack. HOWEVER, in practice, what'd happen is that at random intervals enemies would TRY power attack me with my shield up, and instead of it going into slo-mo/blocking it, it'd instantly cut to cut-scene mode, and they'd impale me (Not a decapitation proc), cut straight to loading screen.
Ok, so what's going on?
Now, remember if the damage is calculated ON HIT, all finishing blows would form a double strike; you land one hit that depletes their health to 0, and then a second finishing blow attack. This doesn't happen, if a finishing blow works, you click the button and you're IMMEDIATLY treated to a close up cut scene of the kill.
What i believe is happening, is that the enemy decides to perform a power attack; if it connects, the attack would deal enough damage to kill me. the operative phrase here is "If it connects". The game has already calculated that the hit would be fatal if it lands, so in the name of dramatics instead of processing an actual attack, it disregards that the player could dodge (due to rapid reflexes), or even that you are currently blocking and skips straight to cut-scene mode to deliver the pre-empted "fatal blow". Your only defence against this is to make sure you have a BIG hitpoint pool, and keep it topped right up, so that the game can never "decide" that the next hit would be fatal and rob you of the chance to defend yourself.
if anything, it's a very good reason to use dual wielding or a two handed weapon instead of speccing into block and packing LOTS of armour instead of using weaker armour and a shield. Because the computer can and will disregard your shield entirely, while conversely if you use a 2 handed weapon you will likely get insta-kill procs against enemies that could've blocked you.
...fix this [censored] plox