Yes I suspect you will run into some problems. There were many issues with DSpace 5.x on Tomcat versions beyond 7. As of DSpace 6 you can use Tomcat 8.5, but I haven't heard of anyone using Tomcat 9. For what it's worth, we are running Tomcat 7 on Ubuntu 18.04 via a binary distribution from Apache's website (
https://tomcat.apache.org/). I deploy the Tomcat tarball into /usr/local and run it from there. This method works on any Linux distribution. I created systemd service units that run Tomcat and it all works really well actually, and I'm running the latest stable Tomcat 7. I can share more tips and configs if you'd like.