I don't really have an opinion on wordpress because I haven't used it more than playing around with it for a little bit and fixing some minor performance issues for a website of a friend. I don't see it used much for larger projects though I know there are some pretty large wordpress installations. Better ask someone that really knows the platform.

I've used both WordPress and Drupal, but like Drupal, WordPress has issues of it's own. In regards to the database, WordPress stores all Post metadata in a single table as a Key Value store. They also don't even typically use InnoDB and zero constraints. Also, the very cult like atmosphere is very strict in not creating your own tables (whatever) and use the tables as they have created them.

That said, I hate Drupal with a passion (I don't get to make the choices at my current place and it's D.O.D Government work) and would rather work with WordPress if a CMS based on PHP is absolutely necessary, their user interface is much, much more friendly - and as a programmer I hate views, panels and all that other cruft I'm forced to work with because "that's the Drupal way."

Edit: Oh yeah, unless they've recently changed it, they store PHP serialized data in the databases, and full urls for the website, so moving domains is quite the fun times and you need a special plugin to do so (even as simple as going from dev.example.com to example.com breaks the serialized strings if you just run some queries on the database). The fact that they store whole urls with domain and everything in the database like that has always skeeved me out.