There are consequences for using the two major tadpoles.
The one that evolves you to half illithid is more of a cosmetic change but an unmistakable warning that you need to stop. And the major tadpole...trust me, you don't want none of what that has to offer...it changes the ending for your character in a massive way...you are doomed to become a normal mind flayer under the control of an elder brain, your companions all leave you, even your love interest, and if that wasn't enough your soul is destroyed...when you die you will be ended forever. It's not even worth letting Karlach use it because while it saves her life in the short term she will be gone forever when she dies...better to let her die now and maintain her eternal soul intact. It's the worst consequence and punishment possible.
I think the main issue is that there's nothing really connecting these choices. They're completely divorced from everything you did up to that point. Using either of the two "major tadpoles" isn't really contingent on using the tadpoles in the past, nor is it contingent on your relationship with the Emperor (or any other character for that matter). When people talk about meaningful choices in CRPGs they tend to expect some level of interconnectivity. They want their decisions to have cascading effects on the world and see that reflected in later choices. If you can pick every evil choice in the game, kill every person you meet and still get the generic good ending then that's sort of bullshit. Likewise, giving into temptation every single time by using the illithid and inserting every tadpole you get into your eye socket only to have the same outcome as someone who never even looked at them just because you rejected the "ultimate tadpole" is an exceedingly hollow choice.
