It works fine for me in with Win 7 host; IIRC it was a bit fiddly at first though. Really choppy UI. Doing the following fixed that for me:
But first: the latest VirtualBox 5.0.20. Perhaps an upgrade is in order? That also then gets you the latest Guest Additions. There is a variety of new stuff in Ubuntu 16.04, so that may make a difference.
Ok, so the issue is likely that guest addition drivers didn't compile correctly.
First, uninstall Guest additions, and purge them from the machine.
sudo sh ./VBoxLinuxAdditions.run uninstall
Second, install dkms. They recommend installing that before the Guest Additions are run.
sudo apt-get install dkms
Finally, install Guest Additions again:
sudo sh ./VBoxLinuxAdditions.run
Note: I've gotten into the habit of moving the VBoxLinuxAdditions.run to home directory, chmodding it to 755, and running it from there. I can't remember why; I think it might have been for a debian thing a while ago. I don't know if it makes any difference, but you might try it that way when you're doing the above steps:
(when you're in the cdrom dir)
cp ./VBoxLinuxAdditions.run ~
cd ~
sudo chmod 744 ./VBoxLinuxAdditions.run
Edit:
Oh, another thing I did: After Guest Additions are installed, you can select its graphics adapter (not enabled by default).
Open Software & Updates, Additional Drivers tab, and under "InnoTek Systemberatung GmbH: BirtualBox Graphics Adapter", select "Using x86 virtualixation solution -X11 guest utilities from virtualbox-guest-x11 (proprietary)". Apply Changes, and reboot.