There is a difference between having an advantage (some kind of boost over your normal activity) and capital-A Advantage (a specific game mechanic from D&D). Larian seems to be throwing out Advantage in any case where a character could be seen to gain any advantage over their opponent, no matter how small, and it's problematic. Giving away Advantage too easily can impact other systems in the game in ways that both mess with balance and tend to reduce some of the interesting aspects in battle.
Giving away Advantage so easily steals the thunder away from existing spells and class features like Faerie Fire, Reckless Attack, and Guiding Bolt. This makes characters that thrive on supporting their allies with things like Advantage a lot less fun to play.
At the same time, it tends to funnel classes with features that benefit from Advantage into one or two strategies that they'll use in every fight. For example, rogues usually need to work a bit to earn their sneak attack. Either they need to coordinate with their allies to distract their enemies or they need to find a dark corner or some obstacle to hide behind. Sometimes it means you'll spend a turn or two just getting in position and waiting for your moment. This is part of the feel of playing a rogue that makes them different from other classes - you don't get many attacks, but if you're smart about choosing your moments, the ones you do get hit hard. They're high-risk, high-reward. In BG3, it's trivial to get your sneak attack every turn by climbing on top of a crate or by walking around behind an enemy. What's the point of having rogues if they're basically just fighters with slightly different numbers?
Most of the time that you're gaining Advantage, you're giving up something else in order to get it. It might take a spell slot or concentration or making yourself more vulnerable to attacks. This creates interesting decisions about whether or not that sacrifice is worth it. Some strategies for getting Advantage work better against different kinds of enemies. Faerie Fire, for example, isn't so useful against high-dex enemies that can make the save easily, but it's great against those lumbering ogres. This means that you have to adapt and change your strategy in each battle; for me, this makes the game much more interesting. Right now, BG3 basically has two strategies - one for ranged attackers (climb up high) and one for melee attackers (get behind them) - and it's really boring.
This leads to the question: if not Advantage, then what? The base rules for D&D 5e have no rules for flanking, backstabbing, or high ground; anything that exists is either a variant option or (usually) someone's homebrew. In my mind:
-Backstabbing should give no benefit at all; characters should be aware of what's going on around them and if they don't see you, you would get Advantage anyway because you are obviously hidden/sneaking.
-High ground should not give a benefit to attacks, save that weapons might have slightly longer range. I could see giving someone on high ground partial cover, depending on the circumstances, but that should be built into the terrain.
-Flanking should give some small bonus. In most of my tabletop games, we give either +1 or +2 to attack rolls with an ally flanking the target.
I'm sure I'm forgetting some other things that BG3 grants Advantage for, but probably shouldn't. Most of these should either be removed or converted into flat bonuses/penalties. If anyone is interested in looking at the math difference between Advantage/Disadvantage and a flat bonus/penalty, there's an interesting writeup about it here:
https://critical-hits.com/blog/2012/06/11/dd-advantage-vs-flat-bonuses/
