I noticed as well that the maven plugin is missing some dependencies. Always run the test with testrunner as well, and then start comparing libs which are in soapui installation against lib used by maven. With -X it should be possible to find out which ones are used.
My current extension of dependencies is:
<dependency>
<!-- not automatic resolved dependency of maven-soapui-pro-plugin -->
<groupId>jgoodies</groupId>
<artifactId>looks</artifactId>
<version>2.2.0</version>
</dependency>
<dependency>
<!-- not automatic resolved dependency of maven-soapui-pro-plugin -->
<groupId>fife</groupId>
<artifactId>rsyntaxtextarea</artifactId>
<version>1.3.4</version>
</dependency>
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>4.8</version>
</dependency>
<dependency>
<groupId>org.apache.xmlbeans</groupId>
<artifactId>xmlbeans-xpath</artifactId>
<version>2.4.0</version>
</dependency>