Eclipse installable units are what you publish to make your Eclipse features and plugins available to your users. You might also want to provide your users with custom Eclipse distributions or products where all your stuff is already installed.
Hopefully, you have lots of tests for your Eclipse plugins, both on the core level (unit tests) and on the UI level (functional tests).
But are you sure that what you published is effectivelly installable in an Eclipse product? Does your p2 repository contains everything that's needed or should your users manually use additional p2 repositories? Could you make their life easier in that respect? Once installed in an Eclipse distribution, will your plugins behave exactly as you tested them in your development environment? Will your custom Eclipse distribution behave correctly once unpacked and run by your users?