Hmmmn, I don't think it's necessarily important for JRiver to have an internal viewer for ebooks & comics as you could always have it opened by with another viewer program. However, what would be especially nice/vital (from the perspective of ebook/comic MEDIA consumers at least) is for JRiver to automatically extract the first page or covers of ebooks & comics (e.g. pdf, cbr, cbz, epub, etc.) instead of just displaying a bland singular filetype, thumbnail icon. It takes too much time individually adding covers for such media one by one. For this reason, I actually have decided not to import most of my comics into the Media Center Library. Rather, I browse and view said local files through Directory Opus with the aid of some plugins -- additionally I use Calibre instead to store and organize all my Kindle ebooks.
I used to be a heavy comic book reader, but not so much now. The issue with comics is that most companies want you to use their own proprietary app or webapp for viewing such copyrighted media. Personally, I just remove the DRM from all my purchased media, and if I can't do that then I move on -- or buy the PHYSICAL media. I've basically lost all my ebooks back when sony stopped selling ebooks. I've also heavily regretted spending almost $2,000 on academic e-textbooks from vitalsource/vitalsoft as their apps are just the worst crap ever... even after almost ten years it's still absolute garbage. As with buying Kindle e-textbooks, it's a better as you can remove the DRM yourself -- although, I haven't tried it with their newer comic books (I have a few) -- maybe test it later.