The web runs on programming. All of the design in the world cannot make your application fast or interactive.
You need software - code - that handles the needs of your application (and demands of the modern web landscape).
There is no substitutefor proper programming.
Code needs to work flawlessly. You need a developer that will properly develop and maintain your site/application.
Writing high performance code isn't that hard for an experienced developer, but experienced developers are hard to find. High performance code means programming that takes the steps necessary to ensure the design is flexible enough for future changes and that nothing is "broken".
This means contingency plans for every route, proper separation, architecture and abstraction, as well as thoroughly testing everything.
Either the dev team finds the bugs or your customers do.
Your customers won't tell you it's broken. Even if they did, it would be embarrassing to have something that is supposed to work... but doesn't.
Poor quality programming/bad developers can harm your reputation and more. There really is no excuse for putting something into a production environment that does not work. There's no reason to develop something unless it is built properly - that's the entire point of software development!