They offer you all their money, your own point for been "evil" aka greed just couple of posts ago. On top of promising to help you later in Baldur's Gate and if you have Karlach the blacksmith will upgrade her power. The cult doesn't offer you anything, everthing is assumed or meta knowledge. The game doesn't make a single hint that you can ally with the goblins (or any reason to do it) until you stumble randomly on Minthara and she talks to you as you know her. At that time you are encouraged by the game on two occasions to attack the bosses and she is the 3rd one(on top of been the main quest), not only in the journal, but ingame tadpole talking. You have to know thru meta gaming that you can ally with them so you either beeline to her and talk, skipping the shaman and the leader so you are not hostile when meeting Minthara. The whole area is terribly designed, placing dozens of quests that can turn all goblins hostile and you are going only by a random journal text that appears out of nowhere that you can raid the grove from where you had to assume you can ally with the cult e.g. another meta gaming.
Greedy for cure to the tadpole thing, and for power, not greedy for 200 gold lmao.
The cult offers both cure and power. And you can deal with the priest, she sleeps you and you are transported to her room. There you kill her after escaping or Raphael sends you help, and she is dead WITHOUT alerting anyone else. Then you can fake the interrogation of the dead illithid in front of the hobgoblin, and there's only 1 combination out of 5 that makes him suspicious and he aggroes. Else, he just lets you pass. And then you talk to the drow, whom you know is there, because people in the camp and corpses you can talk to will tell you. I think even the priestess tells you. The other drow, a corpse in the Druid cove, definitely tells you through Speak with the dead. And, a bit meta, but as the player you see her with the hobgoblins in the opening scene for Act I, after the tutorial.
When I did this whole thing in early access, I had no further knowledge, no guides, didn't look anything up, and it flowed perfectly fine. Helped the goblin escape. Went to the priestess. She tried to dupe me, killed her. Went to the hobgoblin, duped him. Talked to the drow, made the deal. Then, in the end I double crossed them, but I could've just as easily opened the door for them. They are likely to win, especially with my treason, so if you absolutely want to get involved, you get involved on the stronger side.
There's definitely issues, and we've discussed some ITT, but you are exaggerating how bad it is. Its fine. Its actually better than most other "evil" playthrough offerings in cRPGs.