I'm really tired of everyone talking in absolutes (also the overuse of the word terrible). The right answer is (almost) always "it depends".
It depends how critical that functionality is to the core value the site provides. It depends how appropriate it is to a mobile platform experience.
As a contrived example I'd probably be perfectly happy if I could only browse, read, and maybe vote on HN if the mobile experience of reading was significantly improved. I never want to write a comment from my phone, I hate typing on it. Not a great example since a textarea isn't complex and it's easy to add to a mobile layout, I just want to get across the point that not all interaction is appropriate or important on every platform.
This is why responsive design is hard, because doing it well means far more than just changing the layout, size and visibility of elements. It is a different interaction paradigm and generally will need to be treated as such to maintain a quality experience.