It really depends of the implementation for me.

I completelly understand that if you take a mission where you kill a merchant, you loose the option to purchace from them or miss their questline etc. Its a story point where your acts changed the world.

But if you miss some unique loot item from dungeon you can go trough only once, because, it was too well hidden or it was behind some convoluted puzzle that you missed, im pissed.