I was recently asked the question of how applicable Drupal/Joomla are for a large site. Here is my answer:
I have attempted to break down your points to make answering some of your questions easier. I have been developing with Joomla and Drupal for over 2 years and have created around 20 joomla sites and 20 drupal sites, one of which is a 15 site multisite.
Obviously the flexibility afforded by Drupal is a contrast to the user friendly nature of Joomla. Having worked setting up sites for administration by non-IT professionals, I feel that the user friendly nature of Joomla cannot be replicated in Drupal as it does not have a distinct backend, however, if you are able to administer yourself, or offer some sort of training, the additional complexity is worth it.
Drupal is great for sharing database tables or databases, or setting up multisite installations. Joomla appears to struggle with multiple database usage and doesn’t have the same kind of single codebase usage. For administering a number of sites Drupal can’t realistically be bettered (within the same timescale). All you need to do is utilise some customisation of the settings.php file, great!
The stages of both CMSs have differing authentication levels. you have the 7 set in Joomla, and unlimited in drupal however, you should also be able to manage a single user. However, if it is nessecary to move outside the 7 Joomla offers it can be difficult. With regards to workflow, neither Joomla or Drupal work particuarly well as a solely project management platform, but obviously there are modules you can add on.
Version control does work, however, as a lot of the structure is held in the database it can be difficult. Unfortunately I am currently operating on a Windows server which doesn’t have as good CLI tools, however, you could include a MySql dump with your version control commits? You can view revisions of your nodes if required.
I am unsure whether it works completely as AA or AAA compliance, but as it is used for a few high profile sites I wouldn’t expect there to be any issues.
Obviously depends on how much you are loading, but Drupal does appear to load quicker than Joomla, however, I’m sure Googling for speed tests could give you a more thorough evaluation.
Provided you can dedicate the time to it (themes can be tricky) there is very little you can’t achieve. You can also have different themes for subsites if you like.
If I had to recommend one it would very much depend on who is going to be using it. Joomla is perfect for blogging style sites who are administered by non-professional IT workers (I am drawing a distinction as I would assume most capable people could work both). However, for larger scale multi/subsite installations, giving greater interconnectivity across different database I would definately recommend Drupal. (Blimey what a long answer)