I don't know about the rest of the game and I haven't played Grymforge but I haven't felt like I was being rewarded more on ''evil'' playthroughs. That being said one of the ''allures'' of walking the evil path on ANY game is getting more rewards from quests, stronger gear or getting more benefit from situations in general (on some games the way to balance the good path is getting more exp when you're altruistic and don't ask for rewards etc).
This is not a new concept, you get exclusive stuff when you go evil but the same is true for the opposite alignment, that's where part of the replayability comes from.
Who thinks if you take every possible Illithid power you're only going to get rewarded and if you don't use any you're just going to get less content? I think the chances of that happening are very close to 0.
What I think is some people for whatever reason are only able to play good characters and good playthroughs and they're complaining because the ''evil'' path looks interesting and they won't be able to play it, meaning they're going to miss a ton of BG3 content. But thats a ''you'' issue, it's like going to an all you can eat buffet but the only thing you can eat is white bread with butter and you complain that there's too much of the other food and not enough bread.
The argument people are making is closer to "An evil playthrough has entirely additional mechanics/systems that the good playthrough doesn't."
Let's start making that singular, not plural ''1 additional mechanic'', lets add that is actually not an exclusive mechanic, you can interact with said mechanic and still be good (Wyll has made a pact with a devil and everybody thinks he is a ''good'' character, while most people would agree making a pact with a devil is not something good).
Interacting with that mechanic extensively might end up killing people you don't want dead, giving you permanent debuffs, killing you at the end of the game, making the tadpole mind control you to do things you don't want and/or preventing you to fully control some dialogue options...nobody knows.
What we do know for a fact (since the devs have said so) is that going that route will add benefits but it will also punish you. Will that punishment offset the benefits? Nobody knows and on top of that, it is ''mostly'' a subjective thing, some people might think the punishment is far greater while other people might think the opposite.
