It's not just about SEO. There are common aspects to WP and Joomla, but they're not intended for the same purposes. It's the plugins that allows WP to be called a CMS, but they can be unwieldy and like all plugins can fail in poorly designed or DIY themes. I like WP for its simplicity when creating a blog-based site with a handful of "static" pages (which don't have to be static btw) and the easy handling of i18n/l10n, but anything that has a more complicated structure probably means delving into Joomla or Drupal.
WP (4+) has a number of great theme option frameworks, like TitanFramework (poor documentation though!), which I use a lot, and which allows me to create efficient and easy-to-use admin panels for the complete duffer.
I find most WP plugins to be easy to install, activate and use, despite my caveat above. Joomla used to be an absolute pig, although things have improved with later releases. Just a quick my 2p.