The majority of services out there are self-sufficient. Take a very simple http server. It needs to talk to the local filesystem and serve the content requested. The only thing in that equation that might block is the filesystem. Node provides you the tools to deal with that and more. Building an http server in node is enjoyable, easy, and the end result performs well.
The majority of software like a CMS is not self-sufficient. It needs a database, a filesystem, it talks to the http server, it might talk to a dozen other services (generate thumbnails, upload some files to a CDN, scan for viruses in an upload, etc. etc.). You get the idea.
Each external dependency is a blocking dependency. You need to handle it and it isn't easy. Node provides you the tools, but unlike in our first example, where the only thing that blocks is the filesystem, here you have dozens of services that block, with hundreds of pain points. Writing code becomes a chore that is very unpleasant.
That's not the worst of it.
If you miss a blocking operation and don't handle it properly in node, which is very easy, your entire server will go down. In a thread-based or multi-process server-side language mistakes are painful, but not fatal. In node, missing a blocking operation is fatal.
The node guys get this. That's why they're busy trying to cook up a solution around this problem. But they don't talk about it or advertise it.
As of right now, node is definitely not a general purpose solution. It is, however, an excellent solution where you don't have a lot of dependencies that block.