RAW, advantages and disadvantages cancel each others out, always. Here I am interested in the case, where these two (found at PHB - Combat - Unseen Attackers and Targets) cancel each others out:
When you Attack a target that you can’t see, you have disadvantage on the Attack roll.
When a creature can’t see you, you have advantage on Attack rolls against it.
I just don't like how combining these work out. I mean, take an ax and go chopping some incapacitated or prone wood while blindfolded, and see how it goes. And then there are the "abuses" of this, like turning IMHO big disadvantages (fluff-wise, like 600' long bow shot) into a straight roll by creating an area of Darkness, which you can then move behind/into between looking at the enemy and actually taking the attack action. But this isn't the question, but just for the context, explaining why I am asking the following question:
On an upcoming campaign, I am planning to have the house-rule: If you don't see the target you are attacking, you will have disadvantage.
What are the balance and game play implications of this house rule? Will it break something, or have some nasty unintended consequences?