One issue with any site is finding appropriate development resources, or developing them in-house. Since all three CMSes are built on top of venerable PHP and SQL, the barriers to entry at the simplest level are the same.
As an experiment, I compiled a list of freelance development sites, and queried the numbers of developers and jobs available for each platform, arriving at an "competition ratio" – the number of available jobs, divided by the number of available contract developers:
- Drupal: 0.014 jobs/developer (374 jobs, 25808 devs)
- Joomla!: 0.011 jobs/developer (1004 jobs, 91518 devs)
- WordPress: 0.035 jobs/developer (2609 jobs, 75416 devs)
Note: while WordPress wins the ratio test, which may be a good indicator for a budding developer choosing the least competitive/most vibrant market to pursue, the absolute population of Joomla! developers is significantly higher.
The above numbers are by no means a scientific proof of anything, but the general anecdotal implication that Joomla! is slower/clunkier/less SEO friendly than the alternatives, despite a healthy margin of developers, may indicate that average developer quality may be lower than that of the Drupal or WordPress community (and thus, in many ways, harder to find a good developer amongst the crowd).