10
Tricky question.
I'll tell you where Drupal sucks the first time: theming. This is one of the worst surprises you'll get from it. You're making amazing changes on the site without writing a single line of code, then suddenly you have to code a big fat lot overriding some osbcure theming function you didn't know it existed, just to add a linebreak somewhere. Oh, you will love Smarty after that and talk pests about PHPTemplate, no matter how much Drupallers defend it.
Drupal is also heavier than Xoops, doing many more database queries and requesting more memory for some tasks.
But as an application, it gives you all the features you could ever need for your day-to-day job on the site, and the taxonomy+CCK gives you amazing tools to change content structure "on the fly". You'll never get short on features with it.
That's the truth. Breeze on content management, horrible pain for theming. Once you beat theming, everything works like a charm.