I would put forward that both developing and maintaining an application that uses JavaScript is a more complex and more challenging task than developing "classic" web apps.
The reason for that lies with the simple fact that the more heterogeneous technology stack you use the more difficult the work will be.
In this case with JavsScript what we basically have is programming against an automated server output which on its own is a weird idea. The pages of dynamic web applications are generated on the server using some programming language. At the server side you get all sweeties like syntax check, type check, all kinds of automated analysis and refactoring tools are at your disposal.
JavaScript works on the other side of the fence, where there is no information about the big things happening on the server. JavaScript work against an auto-generated markup which is rather volatile and is subject to change. Which is why a developer must pay attention to keep both in sync. The larger an app gets the more challenging the task becomes. And that task is typically a manual work which cannot be automated sufficiently to seamlessly scale with your app.
Therefore yes, JavaScript-rich apps are more difficult to maintain.