Problem: We have to develop a CMS for our sites. The sites are kind of different, but they have common parts. We need to use our external user system and permissions, the menus are different for each of the sites, the forms are posting to a 3rd party system, and we have many other very custom features.

Dilema: Should we use an open source CMS like Drupal, Concrete5 or Wordpress, that we do not have any experience with? Or should we use a framework like Laravel or Symfony that we're good at?

Questions:

1. The core of an open source CMS is better tested. That's correct, but if developing shaky modules on top of it would make the site more insecure than creating the modules from scratch in Symfony? I say shaky modules because clearly it takes time to learn how to develop strong modules on a existing CMS.

2. The open sources CMS's have lots of contributed extensions. Right, but sometimes the extensions are not compatible one to another. Also, when updating the core of the CMS, we have to update these modules as well. From my experience this gets ugly.

3. I'm not sure but I think a custom made CMS is expensive in terms of server load and harder to optimize than a custom one. Is this true?

4. Developing new modules could take longer in a open source CMS, especially when there are lots of custom features. Am I right?

5. Developing new modules could be painful because of various technical issues that may come out.

Are these are valid concerns we should take into account? What would you chose in the aforementioned scenario.

Thanks, R