Not true (Is there a posibility [one in a million] that an app made for bionic doesn't run on juno, yes maybe, but the normal is that any made for bionic will run)
eOS release-juno is based on Ubuntu 18.04, you can use any app that is designed to bionic, in fact we use those repos to install thirdpartyapps where the repo owner doesn't have a specific eOS ...
I was running Elementary 5 in VirtualBox 6 (upgraded from 5) and the GUI stopped working.
Reinstalling elementary-desktop as suggested by @Jeroen using
sudo apt install --reinstall elementary-desktop
did not work for me, but this did:
sudo apt purge elementary-desktop
sudo apt autoremove
sudo apt install elementary-desktop
I'm sure the autoremove isn't ...