My client has a sharepoint 2013 farm with nintex 2013 installed. They did a forklift migration of their content from sharepoint 2010. The moved sites are in a mix of 2010 mode and, in some cases, using 2007 (ui v3) masterpages. Nintex is correctly installed, all features have been activated on the site collection. But, any site not in 2013 mode does not show the option to create a site level workflow, reusable or otherwise. Lists show the option to create standard (not reusable) workflows.