Personally I agree that this check is unfair and completely out of place, especially because it is supposed to be presented as a consequence for using the tadpoles, but due to Larian's mindset towards the evil playthrough to always 'punish the player' rather than respect established lore and player's roleplay for not following their good intended path, this check against ceremorphosis is only present in the evil endings AFTER the player takes control of the Netherbrain and as such is in complete control over everyone's tadpoles.
It is completely nonexistent during the good playthrough's endgame no matter how many are consumed and as such this ultimately feels like intentional punishment to lecture the player, rather than having an interesting consequence with an interesting impact on the story and roleplay.
The solution to keep this consequence fair and appropriate would've been to move this steep check right before the player decides what to do with the Netherbrain. Thus even if the player on a good playthrough consumes so many of them, they would not be magically released from ceremorphosis but instead would have to pass the same check against ceremorphosis before the Netherbrain is destroyed and if they fail, end up living as a mindflayer.
This would actually make it a consequence that respects lore and player roleplay. As it is right now however, yes it's absolutely unfair and completely out of place and I agree the check should not be as steep, especially when the good playthrough doesn't even have a check of its own to pass.
I would argue that the brain is incredibly weakened the moment before you have the chance to kill it. The game makes it clear that even after all the damage you've done and with the help of Orpheus's power, you only have a moment to kill it. I wouldn't be opposed to more consequences for using the tadpole throughout the game though. But I don't know if Larian is going to make changes like that moving forward.
