My issue is not that the player is led to believe that they can find a cure, it's that they cannot actively look for it. If I am playing on a friendship path with Karlach, it just makes my character look like completely uninterested in her fate.
It's all those questions you cannot ask: when you meet the gnomes working on hell engines and ask nothing, meet an experience wizard like Elminster and ask nothing (or Gale, he has been a high level wizard before losing his power, can't you try to research something together?), when Shadowheart gains enough levels to get divine intervention and ask nothing. And then it turns out that a full body transformation apparently fixes the engine. Which maybe, if you researched a polymorph spell, could have helped without costing a soul.
There is a difference between a 'tried your best, yet it was not enough' and 'didn't try much, because why' storytelling. For example, Enderal (a game based on Skyrim engine) has no good endings. But throughout the game the protagonist keeps fighting and looking for a solution.
The moment with the Steel Watchers was one of the worst for me. I didn't see it on my first playthrough since she wasn't in the city with me, so I didn't even know about the connection, but hitting that moment on my second run, it felt so much like the game was yanking my chain.
Like, you're telling me that the Steel Watchers are running on NEW AND IMPROVED INFERNAL ENGINES THAT FUNCTION IN THIS PLANE, and everyone just shrugs and moves on? Just because the robot says that Karlach has an older model? That's not just a long-shot lead like "Can we try to get a Wish spell together?" or "Could Divine Intervention maybe work here?" That was such a direct and practical connection to the exact problem that Karlach has that it is frankly insulting that we can't follow up on that further.
And I say this as someone who really liked Karlach's emotional arc. I loved many of her actress's performances, and even the moment of Wyll volunteering to go to Avernus with her, or the PC's option to do the same, felt like really powerful moments.
Except that in order to create those moments, the game requires you to apparently have blinders on, while simultaneously dangling "The Steel Watchers run on infernal engines that don't overheat" in front of you. The game seems to be aware that you'd be interested in finding a cure, and it's like it has to go out of its way to prevent you from pursuing any path that might lead to it.
Hell, I'm even okay with the idea that the party concludes "Well, if we take you back to Avernus and break into Zariel's fortress, we can fix you there," and Karlach is still reluctant to return to literal hell in case something goes wrong and she just ends up trapped there again. But at least instead of us clawing at the walls trying to find a path forward, it'd be about staring at the path forward and trying to convince her to take it. I don't think I'd be bothered by "Could we develop a variant of True Polymorph to rules-lawyer around this?" if there was already a logical solution on the table.
