While in reality getting ganged up on from opposite directions really is a really bad position to be in and the optional flanking rule might even be realistic, I find that the it just





all over the entire 5e system.
-Got a group with few frontline fighters, or even worse, one tank? Hope your fighters like eating dirt because their armor doesn't matter anymore. Tanks suck under this system, unless the majority of your party is melee.
-Do you play a ranged character/spellcaster? Remain at the normal power level while melee fighters have constant advantage. Hope you didn't get closed in on by two enemies after your single tank got wrecked cause now you're dead even faster than normal.
-Did you build your character to generate advantage? Waste of resources/class features, everyone has advantage anyway.
-Bounded accuracy? Haha oh you... +5 for everyone!
-One big bad boss? Everyone has advantage against him, throwing his current CR right out the window.
-Encounter consisting of many small creatures? Their CR is now way off.
If one were to consider the +2 to hit flanking rules from previous editions to be balanced, what does that say about a the 5e version?
-You can freely rotate around melee fighters unlike previous editions, making gaining advantage trivial.
-Given an average chance to hit, you essentially get a +5 bonus to hit.
-Bounded accuracy increase the value of each point of + to hit even further.
A balanced flanking rule (compared to the value gained in previous editions) would give something like +1 to hit, not advantage.
Don't play with the optional flanking rule, it's dumb and you're breaking your game.