Again, you just don't understand the context of the whole thing here.
- It's not a TT game between D&D geeks, this is a game for millions of players world wide with hundreds of thousands of them not doing even 1 TT game ever or even D&D game ever. People are not born with inherent knowledge of all D&D 5e intricacies - all they will see is 40% hit chance and after couple of battles where they will drool on keyboard they are gone with Metacritic and what not going down.
- What we test now is a normal difficulty, not tactician. It's a difficulty players are expected to tackle when they fire up the game.
- This being a video game - it allows much more complex and bigger encounters unmanageable in TT sessions, difficulty and challenge can come here in other ways than just high AC and winging it with dice and RNG - a cheapest thing to boost difficulty in games.
- Burned actions in turn-based video games are frowned upon as a whole. While people understand you can't succeed all the time, they do expect a reasonable return for investment overall. That's why we can do more in action as is, btw. Last thing Larian needs is just to frustrate people by making them fight excessive RNG all the time. There will be enough of that as is.
- Surfaces in BG3 are more available than in TT and more exploitable too. Creatures need a bit higher HP to compensate that too.
This is the context. Realize - this is a video game, not TT run.
Your argument fails because the PC's have the same armor values and HP formula as the TT rules do, despite the rest of the combat balance being changed - surfaces everywhere, goblins having tons of AoE effects - grease bottles and fire arrows to make explosions everywhere.
As Zorax says, the more they change, the more they have to balance. It's a chain reaction of change, which is only going to make it more difficult.
There are a few encounters they have to balance better. This is one of them. It is almost unbeatable if you don't know what is coming or have a pretty high level. However in this special case I noticed that the song varied in volume depending on where my camera was so I assumed where it is highest is where the enemies are hiding. So I moved my party to a far better starting position without knowing and initiated the battle by being too close to them rather than talking to that boy (which I did in my second playthrough).
I knew it was Harpies the first time. I saw the nest and the piles of bones, I wandered up around the cliffs, and didn't trigger any combat. I saw the kid and knew where the encounter trigger was. I just didn't have any good options.
I considered sticking a character or two only the little ledge, but that's still pretty low compared to the rest of the area, so it wouldn't have fixed disadvantage. I considered sending someone all way around to the top of the cliff, but they would be all alone, which means they'd dead if attacked (or the one left on the ground would be). I just didn't see any good solutions so I walked everyone together into the trigger.
