Value proposition always goes above UI. And above UX as well. You can have dark patterns stacked on top of each other and horrible UX in a product, but if you can have next day delivery — well you’ll jump through all the flaming hoops :)

But if your product does have value, bad execution (both in the overall “idea” and flow of it and in the UI) can lower the positive feelings from your users. Is it enough to kill the product? That depends on a lot of factors. But it certainly will help to have a polished experience.