There is a known issue where WebKit1 and GTK3 does not work with plugins (e.g., Flash). If you need Netscape-compatible plugins to work, use GTK2 or WebKit2.
More development is needed to get WebKit2 support to be feature compatible in uzbl. Currently, WebKit1 is the best option. If you would like to use WebKit2, a non-exhaustive list of things that is known to not work at the moment includes:
- Scrolling
- Injecting JavaScript into the page
- Cookie management
For more specifics, search the README file for 'WebKit1' and 'WebKit2'.