You don't indicate what version of Ubuntu you're running, but recent versions have defaulted to using Wayland in the desktop environment. For Linux guest operating systems, drag/drop doesn't work with Wayland -- you need to be running X11/xorg instead of Wayland in the guest for it to work.
I've just done some testing with a Windows guest VM and a Fedora host (also running Wayland), I'm seeing the same thing. I suspect that the drag/drop problem is due to Wayland. If you have xorg packages installed and disable Wayland in the host, does drag/drop work?
I hope Broadcom can put some work into making drag/drop work with Wayland. Many major distros have already made Wayland their defaults, and the trends show no signs of stopping the move away from X11.