Granted that the PHB describes advantage as being roughly analogous to a +5 or +6 (can't remember which) you suddenly see how flanking absolutely destroys bounded accuracy.
Ok, I know this is tangential to the topic, but… no. The PHB does describe advantage as
roughly analogous to a +5 bonus, and that’s technically accurate, but it’s a massive oversimplification and it leads to confusion like this.
So, what advantage
does is it changes the probability curve of the dice roll from a flat line to a bell curve. It really isn’t equivalent to a flat bonus to the roll at all, but rather, a dynamic bonus that depends on the target number you need to hit. If you need a 10 or 11 to hit, it’s about equivalent to +5, but if you need a natural 20 to hit, it’s worth less than +1. Basically, advantage just decreases the variance on checks. It’s most helpful when your chances are close to 50/50 but doesn’t make much difference if you’re unlikely to succeed in the first place or already had a high chance of succeeding anyway.
Because if this, advantage doesn’t break bounded accuracy. It makes your rolls much less swingy, but it doesn’t actually allow you to hit a higher AC than you could have done anyway. +2 AC might have less impact on the average roll you’re likely to make, but it has much more impact at the extreme ends - the
bounds - of your accuracy.
I'm also thinking maybe Advantage in general just grants +2 instead of multiple dice, to rein in the numbers overall.
That won’t rein in the numbers, it will shift the range of numbers the PCs can and can’t hit, which is precisely the thing advantage/disadvantage and bounded accuracy were designed to prevent.