Powerful npcs should be powerful within the rules. Even if that allows for a tiny theoretical chance that players manage to kill them. (The guard captain being a level 20 redemption paladin is fine, if he has a mechanic like "when you would reduce his hp to 0 your PC dies instead" that's bad).
Peasants and common city guards should be reasonably weak. I really don't like it in some other games, where common guards can easiely kill the PC. Then I'm always wondering, why did you send my hero on the quest to kill the BBEG? Why didn't you just send a few of those guards to do it? Also unlike other games DnD5 heaviely favours the bigger party and has onlv limited amounts of healing and aoe damage. So even if we manage to kill half the city, we will eventually run out of spell slots and get overwhelmed by those low level peasants with slings and clubs.
One thing I really liked about the EA story so far is that there are no NPCs protected by plot armour. If I manage to kill anyone and survive, that might take quest options away from me, but there are still other ways to progress the story. Hope it stays that way.
