Running Nexus Professional in a servlet container or alternative application server is not supported. In fact, you should think of Nexus Professional as an independent server, not as a web application that runs within a servlet container. Nexus Professional is doing much more than a standard web application would ever do. It is interacting with the filesystem and maintaining local caches for hosted and proxy repositories.
Compared to Open Source, Professional is taking on even more responsibility for interacting with system-level resources, and supporting these interactions on several application servers would require a level of targeted support effort that would subtract from the effort we spend to make Nexus Professional the best repository manager in the world.
We provide a war version of Nexus Open Source because end-users are responsible for deployments. If your Nexus Open Source instance running on a Tomcat server is affected by an unknown security vulnerability related to the container, this isn't something we're directly supporting. On the other hand, if a paying customer has an issue with a Nexus Professional server, it is very much our job to jump up and do everything in our ability to support the server.
We've offered professional support for Nexus for years, and we've always strived to strike the right balance between meeting customer's requirements and providing a solid, professional commitment for our products.
We've evaluated a number of containers over the years, and if there was a realistic way for Sonatype to extend support to alternative application servers and servlet containers we would. In the course of our research and development, we've concluded that to do so would lessen our ability to provide direct support to paying customers.