In the post "When should I choose to create a mail app versus an add-in for Outlook?" the author nicely captures the main issue with plugin development for Notes/Sametime using SWT in the fact that any error you make can tie up the client and make the Notes client look like the culprit when it most likely is an add-on plugin that is doing the harm.
"One main difference of note is that the object models were designed to integrate tightly with the corresponding Office client applications. However, this tight integration has a side effect of requiring an add-in to run in the same process as the rich client. The reliability and performance of an add-in often affects the perceived performance of the rich client. Unlike client add-ins, an app for Office doesn't integrate as tightly with the host application, does not share the same process as the rich client, and instead runs in its own isolated runtime environment. This environment offers a privacy and permission model that allows users and IT administrators to monitor their ecosystem of apps and enjoy enhanced security."