Holding "Backspace" deletes only one character

Go to unity-control-center keyboard and make sure keyboard repeat is enabled

or use gsettings to change value:

gsettings set org.gnome.desktop.peripherals.keyboard repeat true

The xset utility can be used to enable keyboard repeat. It works on the X.org level, so should override whatever the distribution is doing wrong.

To enable keyboard repeat, execute:

xset r on

For details, see man xset.

It worked for me on Ubuntu 18.04.