Flanking as listed in the DMG comes as a flat you have advantage as a variant rule, and I've run games that have had that rule used and games that didn't run flanking either.

As a rule of thumb, things began easily to get convoluted between my melee-centric parties, when appropriate 'ganging up' and beating a creature to death was ever present, and dreaded flanking conga lines occur, flanking caterpillars if you would.

Things didn't quite pan out, especially considering if say, a creature was multiple, like a swarm, or had no faces, or even eyes or digits necessary to be in a creature's 'blindspot' as it were.

Large and Smaller Creature Flanking. Now it's fair if each creature of the same height range gets flanking appropriate bonuses off, since of equal measure one's vision or reach is impeded by the arc of your own vision and such. But imagine the difficulty of trying to 'flank' a creature that is of a smidgen smaller than you, it doesn't quite pan out since, well, the attack could come from any angle originally and smaller creatures don't tend to be cornered quite easily. Pair that to Huge or Gargantuan creatures, like giants, dragons, or purple worms that apparently can get ganged up on quite easily by swarms of smallfolk. Appropriate yes, but my argument for giants specifically comes from a place of probably where attacks come from. Like could you really say you gain that much more of a tactical bonus attacking a giant's kneecaps from behind their foot, when obviously their attention is gaze down towards you anyways.

Blindsight Anti-Flank. This is my easy catch-all for negating flanking. Most end-game creatures have it, appropriate 'boss' style creatures, and creatures that lack the vision or facing there of that creatures would abuse to gain flanking bonuses from. Say like an ooze that, apparently if you just attack from any angle with a buddy to the other side, just means you get to 'flank' it, NO.

Say a creature has blindsight, probably approximates they have the werewithal and proficiency to dodge or harry attacks that come from any angle.

Simple and Beneficial Solution. Flanking as Advantage takes away from critical core class features that are easily whisked away if you can just be around a thing apparently.

Flanking as a +2 Bonus approximates for the simplest bonus that can make or break a crucial hit against a target creature, and it equates for say: an enemy toting a shield especially! The most primiere logic, is obviously attacking an enemy where their shield doesn't necessary guard, ergo a +2 bonus is the most generous while being the most considerable.