I accidentally found out that schema files that are new in WildFly 12 haven't been added to org.jboss.tools.as.catalog plugin, for instance jboss-web_12_0.xsd (I'm not sure if there are more).
Why I found out is that I was just trying to verify and close this older JIRA:
JBIDE-25102 Add "jboss-web-11_0.xsd" to catalog
And since then we now have WildFly 12 too.
So this is something to keep in mind when adding a new server adapter.
Also, please check if this needs to be added to https://github.com/robstryker/jboss.org.schema too.