Flanking in prior editions was interesting about as long as Tic Tac Toe remains interesting. After that, it became route and tedious. It was not strategic. It was either going to happen, or was impossible, and either it was automatic and boring (like a Chess Master taking their first few turns)... or it was something that newer players did not understand and was another way for experienced players to tell them what to do. People spent little time between those two unsavory extremes.
As written, in 5E, the flanking for advantage optional rule is nearly automatic advantage. That, my friends, is one of the reason people find feats like Sharpshooter and Great Weapon Master so overpowered. It is because they get advantage almost all of the time.
I played a melee assassin rogue in 5E in a campaign with this rule. It reached 9th level before it made an attack without advantage. Often, the first round advantage came from assassin, surprise or bless, but most rounds it was flanking that gave me advantage. That early experience was one of the main reasons I instituted my alternate rule that gives other benefits.
Alternate rules, such as a flanked creature provokes an OA whenever it moves (unless disengaging) or you can reroll low numbers when you are flanking (a roll of 1 or 2), tend to be less unbalancing.
I've had years of success with the lockdown nature of the flanked creatures provoke an OA (with the additional rule about being able to ignore one creature for these purposes if you give them a special OA not requiring a reaction). It has been very good for my game. It creates significance to motion without giving advantage too easily. It also creates ebb and flow to the combat as people feel pinned down, at times, but can be 'rescued' by the felling of an enemy. This is especially dynamic when you introduce a combat environmental element that encourages movement, such as a flow of lava, rising water, moving walls, collapsing floors, creeping doom, expanding fog, huge slow moving swarms, etc...
