So far, the only other key I have found to not be sent to the Windows host is the WINDOWS key and its combinations, such as WIN+E (Open Explorer), WIN+D (Show Desktop), WIN+M (Minimize All Windows). The WIN-key triggers the openSuse Program Menu. (This is not terrible, just unexpected, and wouldn't necessarily require fixing, as far as I'm concerned. One could consider it a feature since pressing WIN gets us back to the remote guest straight away.)
I am very pleased to report that you can add openSUSE Tumbleweed to the list of supported Linux systems for the RU Beta Viewer! The (Windows) host address book was also successfully imported with saved passwords!
On my first sessions today, however, some keys or keyboard combinations were not passed through to the Windows (7 or 10) remote host, most notably ALT+TAB and DELETE (BKSP does work for character deletion, as did DELETE on the number pad). I did not see a setting in Viewer that would have influenced this.