A war project created by seam gen uses an EntityManagerFactory component. Integration tests using the Embedded EJB container work because the Ejb component starts before the EMF component, but this is by chance only. If the EMF component starts before the Ejb component, it fails. This has been experienced when trying to run a war project on Tomcat.
I'll attach a test that shows this.
I believe the solution would be to add "org.jboss.seam.core.ejb" to the depends list of EntityManagerFactory's @Startup annotation.