My wedding website is through The Knot, and I've had no issues! It's super easy to setup, guests were able to RSVP (and also could change their RSVP if needed) with no issues. I'd definitely recommend The Knot.
Not sure if you’ve considered Mint.com, but that is what my friends and I have all used. It’s also a free platform, but what attracted me was that you can pay $16 for a year and make up your own URL. So it looks good on save the dates/invites and is easy for guests to find. It also has many of the same functions as The Knot, if not all the same.
We went with Zola and haven’t had any issues. I don’t like a lot of flowery or busy backgrounds with designs (especially since none of them truly fit our wedding style/decor/colors). But Zola had a simple black and white template that I liked. Also it’s easy to make changes from the app. However, we aren’t using them for registry since we only registered with Amazon.