Sneaking is op because you can start a battle with a sneak attack from Astarion and then the combat tracker is just Astarion and the enemies that saw him, or maybe only the little pillar of light that shows up.
Once Astarion gets locked into the combat tracker you can still maneuver everyone else around the enemies sight lines and get a few more alpha strikes in outside of the combat tracker. My paladin – in full plate – can walk right up to an enemy that’s frozen in the combat tracker as long as he doesn’t walk into the sight cone.
I beat dror, m, and gut in the goblin camp all without any of them taking any turns because I was able to alpha strike them into the dirt by just starting combat with the whole party sneaking, and then walking them right up outside of the combat tracker.
Is there a party that can really exploit this? Ranger, wolf barb, shadowharts +10 party sneak, and an assassin rogue? Just alpha strike everything.
Should noise and peripheral vision be things? Yes. Are they well defined in 5e? No. Is the game capable of being a great stealth game with the engine? Also no.
The biggest issue is the preciseness of the field of vision. It could probably be wider once combat has begun, but also there could be a gray area around the red where you don’t know if they can see you.
You could use:
Modding community will come up with some fun stuff. I’m pretty excited to see where that goes in the next few years.