There are a lot of things to worry about when you’re building a website, whether or not the system you’ve chosen to manage content will scale properly shouldn’t be one of them. Most development shops, those I’ve dealt with anyways, have a go-to CMS they sell to their clients because in most cases the salesman isn’t responsible for development. That’s fine 90% of the time but every now and then you run into a project that might have been just a bit easier build in Joomla, or maybe a bit more user friendly if it was Wordpress.