Joomla is used by half of our small team, so roughly 4 people, all with varying levels of CMS experience and Joomla-specific knowledge. This was a legacy platform used by the firm and we have continued to use it as our primary website CMS.
Various extensions are not always compatible with one another
Overall UI and UX of the CMS itself is confusing and often takes way too much time to ramp; it's not very "user-friendly" to borrow a buzzword, and it often takes new team members longer than anticipated to ramp up and feel comfortable using the platform
Lack of safety and security features to protect site content and user/member information collected through the platform
Joomla! is best suited for developer-types and those with more advanced knowledge of navigating CMS systems. I find it difficult to manage SEO-centric websites as opposed to ones with more visual assets. More creative departments or marketing teams may not be best suited for Joomla! and may prefer a more user-friendly interface like WordPress.