Don't Use Flanking

A common house rule is flanking -- and it sucks

It rewards/encourages/snowballs bad encounter design

Basically, flanking gives a significant advantage to whatever group has more characters/units. A typical boss fight might involve only the boss and one or two adds; since the party typically outnumbers the enemies in this situation it creates a rewarding/memorable experience for the players (and also shortens the encounter... probably not in a good way)

It punishes melee characters even more

This might seem counter intuitive because it gives melee users more sources of advantage. Yes, that's absolutely true but let's talk about a somewhat common situation. The party is a fighter, cleric, wizard, and rogue (ranged) against 4 fighters. What happens? The fighter either runs in and gets ganged up on by 4 fighters or doesn't do anything because it doesn't want to be flanked by 3-4 enemies. The cleric doesn't need to go into melee because it can use Sacred Flame from a safe distance so this combat basically prevents the fighter from participating in combat (when that's exactly what a fighter wants to do) or they throw themselves in danger and forces the party to bail them out.

Advantage basically works out to an extra attack, it's a little worse (~88% chance to hit, assuming a typical 65% chance to hit) and this starts to get really bad for the melee characters as more attacks are added.

It punishes class design

A lot of subclasses have ways to generate advantage on their own and some classes (barbarians and rogues) have features that grant advantage at a cost. With flanking, those classes lose a lot of value (or at best, decision making because a barbarian will Reckless Attack anyways since they already have disadvantage through flanking).