Well, I both agree and disagree with you.
I agree that the main plot revolving around the tadpole and ceremorphosis is terribly underwhelming and all over the place. There are remnants of a better plot and the consequences everywhere and it makes it worse.
Having said that, everything else you are free to handle or mishandle as you see fit. They weren't afraid of letting you fail but soldier on. It's only the main plot that is devoid of choice.
Don't do the quest to help the Tieflings / stop the ritual? They're doomed and you'll never see them again.
Fail the strength rolls to get Gale of the portal? You will go through the game without him.
Stake Astarion? That kills him. Permanently.
Don't intervene between Shadow and Lae'Zel? One of them dies. Again, permanently.
Allow a Hag to operate on you? Say goodbye to your pretty face [for some reason this isn't the case for Volo's surgery though]
Fail to Guard Isobel? All of the Harpers die.
Kill Mizora? Wyll's a goner.
