The latest release of WinRAR by default uses RAR5 compression format which is not compatible with most comic readers at this time. Any .rar or .cbr file created with the current release of WinRAR will be unreadable on .cbr readers. (Files created with older versions of WinRAR will still be readable.)
I recommend anyone not still using an older version of WinRAR to instead use 7-zip .zip compression and create a .cbz file instead.