As a web collaboration and publishing platform, Tiki Wiki CMS Groupware has encountered the challenges of both wiki syntax and WYSIWYG, and has implemented solutions.

Wiki editor
Easy to learn With WYSIWYG, it's easy to get started and do simple things, but more complex styling and layouts are difficult, if not impossible. With Tiki syntax, there's a small learning curve to get started, but the path to doing more complex things is quite manageable.
Extensive browser support Even text browsers are supported.
Readable source by default No extra plugin is needed.
Secure WYSIWYG editor is dependent on host platform/website to provide user rolls, etc. On the other hand, with Tiki, editor permissions can limit to text input only, for example. Tiki's post-installation plugin-free approach avoids external code and its possible vulnerabilities.
Flexibly extensible Tiki's toolbars can be extended by site administrators and editors, to add new buttons that produce any needed styling or layout.
Future-proof Tiki's pages can be exported in wiki syntax (plain text) or HTML format.
Fast page load With the wiki syntax editor, less overhead means faster page loading.
Mobile-friendly Tiki's wiki editor is compatible with all mobile web browsers.

Restricted to available elements and styles Add tools to the toolbar if available, or use wiki syntax instead, which allows more freedom.
Relies on JavaScript or Flash so may have security vulnerabilities Use of WYSIWYG can be restricted with user and object permissions.
Not easily extensible If WYSIWYG isn't adequate, in Tiki you can use wiki syntax or HTML instead.
It's sometimes hard to produce the specific layout and style that you want HTML syntax can be entered directly, using Tiki's HTML plugin, to create any layout and style.

Wiki syntax
There can be a learning curve if the author needs to input the syntax manually. Tiki has editor toolbars, input forms and help screens to minimize the need to remember and enter syntax manually.
There are a number of variants of wiki syntax, so syntax might need to be relearned from one editor/platform to another. Tiki's syntax doesn't need to be "learned" if the editing aides are relied on instead.