Can you elaborate on/provide a sample of the "large number of mysterious failures". It might help. There are tons of folks using the JBoss/Tomcat/Struts combo. We have run 100s of thousands/millions of requests through our JBoss/Tomcat servers, all through jk2, and all our apps use Struts. There have been problems, but none related to JBoss. All self-inflicted wounds in the apps.
JBoss has little to with an HTTP request bound for Tomcat (aside from security and classloader). Tomcat listens on port 8080. Tomcat services the HTTP request. Tomcat invokes the Struts action. That doesn't change when you go to integrated JBoss/Tomcat.
Are you using the same version of Tomcat and Struts as before?
We've seen this kind of behavior in the past when using mod_jk2. The reason is still not well identified but it is definitely not a problem caused by JBoss AS.
That's why we recommend using mod_jk1.2 instead as explained in this wiki page
FYI for those that don't know. Apache 2.1 is going to include a replacement for JK called mod_proxy_ajp