Under the circumstances described below Vaadin goes into compatibility mode (bower) despite the lack of bower resources described in the pom.xml. This is not a general problem with Wildfly as deploying a non exploded war (the result of mvn package) works perfectly fine.
To me this bug is dealbreaker for Vaadin 14 and npm as this setup provided me with hot code and resource replacement.

$ mvn clean package on the project
1 Adopt version:
openjdk version "11.0.3" 2019-04-16
OpenJDK Runtime Environment AdoptOpenJDK (build 11.0.3+7)
OpenJDK 64-Bit Server VM AdoptOpenJDK (build 11.0.3+7, mixed mode)
2 DCEVM version:
HOTSWAP AGENT: 14:32:37.612 INFO (org.hotswap.agent.HotswapAgent) - Loading Hotswap agent {1.3.1-SNAPSHOT} - unlimited runtime class redefinition.
HOTSWAP AGENT: 14:32:37.857 INFO (org.hotswap.agent.config.PluginRegistry) - Discovered plugins: [Hotswapper, JdkPlugin, WatchResources, ClassInitPlugin, AnonymousClassPatch, Hibernate, Hibernate3JPA, Hibernate3, Spring, Jersey1, Jersey2, Jetty, Tomcat, ZK, Logback, Log4j2, MyFaces, Mojarra, Omnifaces, Seam, ELResolver, WildFlyELResolver, OsgiEquinox, Owb, Proxy, WebObjects, Weld, JBossModules, ResteasyRegistry, Deltaspike, GlassFish, Vaadin, Wicket, CxfJAXRS, FreeMarker, Undertow]
Starting HotswapAgent 'C:\SDK\Java\dcevm\dcevm-11.0.1+8\lib\hotswap\hotswap-agent.jar'
openjdk version "11.0.1.6" 2018-12-16
OpenJDK Runtime Environment AdoptOpenJDK (build 11.0.1.6+8-201904021629)
Dynamic Code Evolution 64-Bit Server VM AdoptOpenJDK (build 11.0.1.6+8-201904021629, mixed mode)

This is basically a simplified version of this ticket (#6073) as I initially thought the problem was caused by some more special circumstances.
Does the problem get solved if you ask Eclipse to refresh the project files (F5) after mvn package and before adding the module exploded to Wildfly? The problem seems to be that flow-build-info.json (which configures npm mode by setting vaadin.compatibilityMode to false), is not present in $WILDFLY_HOME/standalone/deployments/my-starter-project-2.0-SNAPSHOT.war/WEB-INF/classes/META-INF/VAADIN/config/.
Yes, it does!
But as soon as I update the maven project manually from within Eclipse the project is broken again.
Refreshing doesnt help from then on.

We are currently looking into how to reliably detect the right mode to start in. As a workaround, after updating the maven project in Eclipse, you can manually run maven prepare-package via run configuration. In this run configuration, enable Refresh -> Refresh resources upon completion, then Wildfly deploymet will be republished, picking up the flow-build-info.json.
Thank you for replying this quick.
Ill give it a try tomorrow.
From Flow 2.0.6 (platform 14.0.0.rc8 once released), the Vaadin Maven plugin automatically runs prepare-frontend goal (which in turn copies flow-build-info.json to the deployment classpath) when the project is updated in Eclipse.