You're almost always dependent on perfect code to keep your site running, be it server side or client side. If code breaks on the server side you're just as screwed.
The key difference is that the JavaScript approach makes you dependent upon third party ads being served correctly, while the standard HTML approach makes bad code in third party ads just affect the display of those ads.
That third party dependency is a significant difference.
You're almost always dependent on perfect code to keep your site running, be it server side or client side. If code breaks on the server side you're just as screwed.