This is based on the text in Page 5 of Xanathar's Guide to Everything:
Even if more than one factor gives you advantage or disadvantage on a roll, you have it only once, and if you have advantage and disadvantage on the same roll, they cancel each other.
This would imply that an Assassin Rogue, attacking from Stealth, at a target within 5 feet who was Prone and hadn't taken a turn in Combat yet (3 circumstances that grant Advantage, which I will refer to as "instances granting Advantage"), would still only roll normally against a target who happened to be wearing a Cloak of Displacement (one "instance granting Disadvantage"), and thus wouldn't get Sneak Attack damage.
While this probably simplifies "big" encounters where there are a lot of instances granting both Advantage and Disadvantage, it seems to not reward strategic play at all, and vastly increases the power of the Cloak from being a generally-good tool for preventing getting hit for a few turns to entirely countering Sneak Attack on the first turn, meaning that, compared to a Player, an enemy never has to worry about being Assassinated (1st Turn only) or Sneak-Attacked until their later turns, especially since there is no way to Delay a turn in 5e, which means that, if an Assassin Rogue ends-up going first, they can either move and then Ready an Attack (which is a rather clumsy way to work this) or just attack and lose Assassinate.
I know some DMs whom, instead of just ruling that even one instance granting either counters every single instance granting its counterpart, will count each instance granting Advantage, and compare that to each instance granting Disadvantage, and determining which remains after all have been cancelled.
In essence, it can be distilled to the three mathematical expressions, where a is the total of instances granting Advantage, and d is the total of instances granting Disadvantage:
$$ \begin{align} a - d > 0 & → \text{Advantage} \\ a - d = 0 & → \text{Normal} \\ a - d < 0 & → \text{Disadvantage} \end{align} $$
In the case of the above example with the Assassin Rogue, this would mean that, after the single instance of Disadvantage cancels with one of the three instances of Advantage, meaning that the attack ends-up going-through with Advantage because \$3 - 1 = 2 > 0 → \text{Advantage}\$.
How does this houserule change the balance of the game?