In case of Drupal, it has Fewer developers and more expensive developers.
In case of Joomla/ Mambo :
1. Nasty, nasty core code. Very difficult to fix broken items.
2. Fractured community (never healed after Joomla/Mambo split back in 2006).
3. Most good plugins are pay.
4. Developer pricing is all over the map as there are many old-school Mambo/Joomla developers still ought there churning out convoluted future-resistant code quite affordably.
In case of Wordpress, it has very fast an upgrade cycle. You have to keep upgrading your site, whether you like it or not, for security reasons. There are no security releases only new versions. Feel the pain for a commercial site with running a full complement of plugins. Corollary: choose your plugins and plugin developers very, very carefully for cleanliness of code and frequency of update.
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