Darkvision really has no impact at all.
LOLWAT????
You really don't have any idea do you?
For characters without darkvision, its the only buff I care to give them from a camp characater.
Try playing on tactician and having disadvantage stacked with darkness penalty, you will never be able to hit anything without darkvision.
Basically every location is conveniently lit up by some light source. Someone kindly goes around and lights torches, candles and braziers in every dungeon, cave and tomb across Faerűn. Even in the rare instance that they are not, that is easily overcome by the sheer amount of items, potions and scrolls that you are showered with.
If you have encountered a fight where darkvision had an impact I'd be interested to know which one, if you can put it under some spoiler tags. Because I explored all of the underdark with a ranged githyanki character on tactician and almost never suffered from disadvantage (and when I did I still had 90% chance to hit).
