Most of the "niche" renderers use the same codebase (or at best only rely on large common libraries like FreeType); the JailbreakMe 2/3 exploits, for example, were able to affect almost all of them (its payload simply assumed it was trapped in MobileSafari and wanted root on an iPhone, but on systems without codesign I believe it could have affected just about everyone fairly generally). The thing that Adobe has going for it is that when there is a bug, they tend to fix it, they fix it correctly (JailbreakMe 3 shouldn't have been possible), and the fix is pushed quickly to users via automatic update prompts.