I just learned that RTF display has been fixed in a recent version of iOS 8 (I'm not sure which version), but the background is now transparent. So if you are viewing RTF files with the dark color theme, the text might be invisible. If you change to the light color theme, you should see the RTF file again.
This is how Word files have worked for a while, and the app addresses that by placing a light background behind Word files in the dark color theme. I'll update the app to do the same thing for RTFs in the upcoming version 4.1, so you can use RTFs with the dark color theme again.
I believe that resolves all the iOS 8 related issues.