This is a tough one. I take your point. At the same time one of the things I love about RPGs is the steady aquisition of new stronger abilities. And I love developers who are in on it.
Not to mention I think a messed up look would be quite a high price. I wouldn't want that.
If it's something like playing as an evil character absolutely. I would expect violence and hate. Consequences. For something that really boils down to a way to get new powers I don't mind if the game winks and tells me I'm on the list. I would be very frustrated about the other things Laurentia mentioned which I'm going to pretend I didn't hear but not this in particular.
I will say I'm suprised if someline like Lae'zel with her feelings about mindflayers stays with you if you keep absorbing tadpoles.
You get the stronger abilities already with level ups, so that base is covered. I just think, using the tadpoles should have some kind of consequences.
In EA, when you used your tadpole powers, that dream visitor became really creepy touchy feely and your companions are pissed at you. And, as mentioned, other True Souls, like Nere, could dominate you.
I don't think, something as powerful as the tadpole powers should come without consequences.
